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Imagine an evil dictator. He imprisons his enemies in a labor camp, but they never do any work! He doesn't want to torture them, or force them to do that since he is using all of his soldiers to build him a beautiful castle. Instead, he decides to create an incentive: every time you do a certain task (such as carrying 500 stones a total of one mile), you get a coin from an automated machine. If you get some huge number of coins, you earn freedom!

The only problem is that he needs a door to release exactly one. He can't afford to send a soldier to guard the door, so he needs a door that will allow exactly one person to go through when a token is deposited into a coin slot. This is going to be based off of medieval-style technology, so no complex scanners or anything electronic.

I have thought of:

  • A revolving door (But two people could both fit in)

  • A weight based door (Two thin or young people could get out)

  • A scanner which would check for people (Too high tech)

I've wondered about this for a while, just out of interest,

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    $\begingroup$ It's easy to make a revolving door where two people can not fit in. Similar devices have been used for a very long time - they were originally called called "turnstiles" but they can be made like full-height doors. Some pictures here: en.wikipedia.org/wiki/Turnstile $\endgroup$ – alephzero Oct 2 '18 at 21:08
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    $\begingroup$ The question us invalid. You are creating an incentive structure where a hierarchy of prisoners would form. Those at the top would tax your coins and use them to make their own comfortable existence, eventually resulting in a mirror of the outside with armies and lords. The thugs at the top would let just a few out to keep the incentives working, adjusting the freedom rate per economic conditions like a central bank. In the end your internal army would prevail over the external. $\endgroup$ – Sentinel Oct 2 '18 at 21:51
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    $\begingroup$ Um, isn't this a turnstile? Or, I guess, you could go more complicated with a mantrap-style setup, but what am I missing here? $\endgroup$ – HopelessN00b Oct 2 '18 at 22:03
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    $\begingroup$ @Sentinel while you're probably right about the social effect, I'm not sure that invalidates the question. Addressing that effect is probably a good idea, but people enact imperfect systems all the time and unintended consequences are everywhere. $\endgroup$ – Morgen Oct 3 '18 at 14:00
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    $\begingroup$ This doesn't sound like a very evil dictator. Allowing slaves to earn their freedom? Not wanting to torture them? Not using his soldiers like cattle? $\endgroup$ – trysis Oct 6 '18 at 14:46

41 Answers 41

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Put it another way:

Just let the prisoner out when he/she pays for his/her weight.

The prisoner opens the door, gets in, and closes the door. The door is automatically locked. The inner walls are all slimy/oily and slant inward at the top, so he cannot get a hold on them. There is a slot for coins. He/she puts one big coin for each pound and when the required number of coins have been inserted, a hidden lock clicks, and an exit door on other side unlocks for a minute and he/she can walk out.

If the required number of coins are not inserted in the slot within a minute, all inserted coins are eaten (lost) and the input door is unlocked. The prisoner can go back and work harder to try again next time.

This also prevents locking the box forever by a malicious prisoner. Until the input door is closed and locked, all coins inserted in slot are just eaten, so do not even try to block the input door from locking.

The output door is only unlocked for a short time, then closed and locked again. Until it is locked, the input door would not unlock. The exit path from the output door is oily too, so you are forced to slide away and cannot help others somehow block it. Even if you could, that would just keep prisoners in camp so no harm to the dictator and an occasional patrol can simply put the offender back in the camp - with the angry mob he/she blocked.

The output door slides up, has a sharp bottom, and is heavy - trying to block it with your body would result just to being guillotined into two pieces. The inner piece would get cleaned by other prisoners soon (nobody would like to pay for half of someone else's corpse), the outer piece will slide away.

Dictator gets a coin for each pound of prisoners body and cannot be cheated. Even if two or three prisoners came in, they have to pay for their cumulative weight, so no cheat (other than reducing the rounding-ups) possible.

(Well, just be sure, that prisoners do not have tools and material inside, so they cannot sabotage it or dig holes through or make ladders to climb over the wall of the camp. But it is not specific to this kind of exit; this is common sense in unwatched camps.)


I am an evil dictator. After all, the 1 minute time is really short for inserting all coins :) No, really, I am not used to count in pounds. Maybe the timeslot is more like 5 minutes, or 1 minute after last coin inserted, maybe there are 10 and 100 pounds coins or something like that ... just the prisoner is not allowed unlimited or too long time inside ...


@theRiley: Technically it does not ensure "just one person", but as the OP put his question, his "evil dictator" have no need for extracting "just one person":

  • After all, he does not kill or torture his captured enemies, but he wants to free them instead (so there must be some feeding for them and really good one and stable, to ensure, that everyone has enough strength to make a lot of hard work, if he wish - and in medieval environment, many of the prisoners would be poor peons, which would like stay in the prison forever, just for the food - as work is totally optional - it would be better for them, then be "freed" to everyday hard work and big chance for starvation on regular basis)
  • Also that means, that the dictators peons have to provide food for prisoners - for free
  • He even allows them to self-organise and left them totally unwatched in the "prison" (or more like reacreational camp for big part of them?)
  • Also he allows them to make weapons and tools at their will (at least at stone-age level)
  • Work is purely optional and food does not depend on that - there must be an abundance of food
  • Even if they want some more materials - there are already bones and skin available (the evil dictator usually rules more than one village - sacrificial lamb could be chosen from those, who the prisoners does not love, or even hate - who said, that I cannot kill a prisoner from another country - is he/she even human for me?)
  • He provides infinite source of stones for making more tools
  • Well, we can even try to step over the stone age, as we can use metals from the coins (yes expensive in work, but totally possible, if anybody can earn "some huge number of coins, for earn freedom")
  • He allows his "automatic machines" containing precious coins just free to attempts to rob it (with lot of stones conveniently at hand) (and I can make holes where I wish with bow, bone and sand)
  • Also prisoners can use the stones to make stairs (or just hill) against any wall and run away.
  • He push ALL his soldiers away to work on his castle, not even left the smaller peon to go to prison once a week just to press "confirmation button" to say, that only one - not two - prisoners are leaving on one pay.
  • The pay can be distributed between prisoners at their will (just exchanging coins for say services, lost bets (it is easy to make cards/dices/other gaming properties in such a camp) or just bullying and rocketeering)
  • He can (and is willing to) pay for automats evaluating his coins as not fakes
  • He do not care to rule the prisoners, watch them, put them "in line" or something like that, he even do not care, if he has ANY prisoner at all (everyone can work and go free), but if he have any, he makes sure, that the prisoner lives far better than the average person in that time.

So in the light of all it - why he needs to release just one prisoner at time? The only logical answer is his pride, as he does not want to be cheated and seen as a fool (he is not seen as evil by prisoners anyway - more as really a benevolent and fair ruler, who gives food and games for free and allows you to leave, if you want and do some tedious work fork for him).

So this answer does not concentrate on exact wording of question, but to deliver, what our fair ruler (calling himself evil dictator) really wants - make sure, that no prisoner can cheat its exit due exit gate, without paying "some huge number of coins".

This answer shows, how to make it automatic, fair, hard cheatable and without even one sentient guard/advanced technology/magic.

None of the other answers so far provided a sure solution for a uncheatable one-person only, and I am afraid, that in the given restrictions it is impossible.

So I conclude, that what the dictator needs are satisfied by this solution and that satisfying the evil dictator inside the restrictions on low technology and no guards was the core of the OP's question, even if it does not follow the text of question to the last letter, as it solves the problem hidden in the question.

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    $\begingroup$ As a side note, this also incentivises the prisoners to starve themselves so that they weigh less and therefore have to pay less. Just a little bonus suffering. :) $\endgroup$ – Ethan Field Oct 2 '18 at 15:23
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    $\begingroup$ Yes, also keeps the price of food low :) $\endgroup$ – gilhad Oct 2 '18 at 16:39
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    $\begingroup$ @EthanField Or they take the fast route and chop off limbs $\endgroup$ – Hagen von Eitzen Oct 2 '18 at 21:35
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    $\begingroup$ @HagenvonEitzen Imagine working as hard as you can to make enough coins to escape without an arm, chopping it off only before entering the room and finding out you're one coin too low so you have to work with one arm now. $\endgroup$ – Hankrecords Oct 3 '18 at 9:28
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    $\begingroup$ This brings a new meaning to "cost an arm and a leg". $\endgroup$ – March Ho Oct 3 '18 at 14:17
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Just let 2 out if they manage to fit. The door only leads to the pit of despair, after all.

I mean you can't actually be freeing them. They might then be able to provide information to those on the outside about things that might compromise the prison's security.

So let 2 get out on occasion - the joke's just going to be on them.

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    $\begingroup$ This is the obvious answer -- Lieing to pathetic slaves is like, evil overlord 101. $\endgroup$ – Sidney Oct 2 '18 at 17:43
  • $\begingroup$ This answer looks to be the same as this one $\endgroup$ – Cort Ammon - Reinstate Monica Oct 3 '18 at 19:42
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    $\begingroup$ I fail to see how this answer the question: one man door. $\endgroup$ – Vylix Oct 5 '18 at 17:04
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What about something like this? door

Allows the door to open just enough for 1 person to enter space and have to close the door to enter the room or outside. It could be very tight with spikes around the walls so they don't try and squeeze in.

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    $\begingroup$ I'm almost certain that, if I was a slave, I would at least try a few times to squeeze in, EVEN if I have to be punctured a few times ! $\endgroup$ – Don Pablo Oct 2 '18 at 7:46
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    $\begingroup$ I could fit at least six children in there. Eight if I can break bones. $\endgroup$ – PStag Oct 2 '18 at 10:33
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    $\begingroup$ Just curious where that kind of door is used, I've never seen something like that before! $\endgroup$ – BruceWayne Oct 2 '18 at 14:09
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    $\begingroup$ @BruceWayne They are used in many places where you have access cards and a high security standard. We have them at work in our datacenter, for example. $\endgroup$ – Philipp Oct 2 '18 at 14:53
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    $\begingroup$ And now I'm wondering how feasible climbing through the top of the door's gap would be. $\endgroup$ – Baldrickk Oct 2 '18 at 15:51
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A revolving or sliding door with a human-shaped "mold" will do the trick.

A problem with traditional turnstiles is that two people can easily squeeze into the space, by sandwiching up or sitting on another's shoulders. If you just make this space smaller, bigger people will not be able to pass through.

This problem can be somehow mitigated if a passing person would have to fit into a "mold" that is designed to fit one large human. It is much more difficult to pack two smaller humans into a seemingly sufficient volume if this volume is SHAPED.

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  • $\begingroup$ I like this idea! $\endgroup$ – Redwolf Programs Oct 2 '18 at 1:21
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    $\begingroup$ Which "bigger people"? We're talking a medieval labor camp. After a few weeks, everyone will be slim. $\endgroup$ – Tom Oct 2 '18 at 8:00
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    $\begingroup$ "A revolving or sliding door with a human-shaped "mold" will do the trick." Uh... Engima of Amigara, anyone? $\endgroup$ – Adonalsium Oct 3 '18 at 18:45
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    $\begingroup$ @Adonalsium DRR…DRR…DRR $\endgroup$ – Bob Oct 4 '18 at 3:01
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    $\begingroup$ This got me thinking about other ways to limit. How about only allowing one foot out. So not a door but a foot manacle that would clamp on to the prisoner's leg, lift them and carry them over a pit of lava. Perhaps a second prisoner could hold on to the first prisoner, but not if the mechanical manacle picked up the prisoner from a vat of slippery oil. $\endgroup$ – joeytwiddle Oct 8 '18 at 5:40
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Have you considered a psychological barrier?

It's medieval times, just tell all of the prisoners that there is a mechanism in the system - there are two doors joined by a hallway. You go into the first door, and it has to be closed and locked again before the second door unlocks. Tell everyone that there is a trap door (to a deep pit with spikes) set to open if there are two people in the hallway between the doors at the same time.

Any prisoner who has earned their freedom isn't going to want to risk death to let another person in/out with them.

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    $\begingroup$ Perhaps make the mechanism actually work ... but controller by an unseen guard pulling a lever. After a live demonstration or two, the guard doesn't actually need to be at the station anymore for the threat to stick. $\endgroup$ – ThunderGuppy Oct 2 '18 at 20:08
  • $\begingroup$ @ThunderGuppy The OP specificially said they don't want to have to have a guard monitoring the system. $\endgroup$ – Pharap Oct 3 '18 at 13:17
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    $\begingroup$ @pharap Because of cost concerns, a permanent guard isn't viable, but I'm proposing one who's only there for a short while, just long enough to establish the illusion of the reliable pit trap. After that, they can be fired ... or killed, if that's your villainous speed. $\endgroup$ – ThunderGuppy Oct 3 '18 at 13:20
  • $\begingroup$ @ThunderGuppy Or chucked into the prison. Who's gonna believe there's no replacement guard? $\endgroup$ – Bob Oct 4 '18 at 3:03
  • $\begingroup$ Even better if the slaves inside can't actually check if the people who went in actually made it out. That way the threat is ever present $\endgroup$ – Miguel Bartelsman Oct 6 '18 at 12:25
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Human Trebuchet

Your an evil dictator, the safety of a freed slave shouldn't be too high a priority. Sling them into a nearby lake with a human trebuchet. Strap them to the device with a bracelet or manacle. If someone else decides to loosely attach themselves to the newly freed individual they will likely die on launch. If they strap themselves tightly they will drown once they hit the water.

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    $\begingroup$ I like catapultes. I like trebuchets much more. I really like your answer ! $\endgroup$ – Don Pablo Oct 2 '18 at 7:49
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    $\begingroup$ you would need a guard to man the trebuchet. $\endgroup$ – theRiley Oct 3 '18 at 14:27
  • $\begingroup$ @DonPablo You should like catapults more. They're the superior siege weapon, capable of slinging a 90kg projectile over 300m. $\endgroup$ – Adonalsium Oct 3 '18 at 18:46
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    $\begingroup$ @Adonalsium isn't that trebuchets? I'm convinced that's trebuchets! $\endgroup$ – Don Pablo Oct 3 '18 at 19:05
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    $\begingroup$ I had a brain fart and now I can't edit it. Oh well. Joke fall flat. $\endgroup$ – Adonalsium Oct 4 '18 at 12:37
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I think you may have over-constrained your poor evil dictator. A dictator who runs a labor camp without a single guard or employee in the camp is in a really strained position. Usually labor camps have dozens or even hundreds of employees, because you generally want to have an eye on these dangerous individuals. That's actually the point of labor camps. It's a way to keep your eye on people that you don't want to kill immediately.

As such, I don't think the dictator really need to have this precision. He doesn't need to worry so much if one or two people get out.

He doesn't have to worry because he makes the door as a revolving door. No windows like you'd see on a hotel revolving door. Just a solid blank wall that rotates. He puts a ratchet on the door to make sure it only revolves in one direction.

If you've seen the hotel revolving doors, there's a period where you can't go in and you cant go out. This is important because the purpose of these revolving doors is to keep air from rushing in from the outside, causing the hotel heater or air conditioner to work very hard.

Well, once you reach that point, you realize that the door is opening over a spike pit that will seal your doom. There was no freedom. This is an evil dictator, not an honorable master. If you try to stop and wait, so you don't fall into the pit, your friend who's behind you with his token will try to rotate the door and push you in.

As such, given the dictator can't even spare a single person to watch over this labor camp, I don't think he'll mind if 2 people die instead of 1. He clearly doesn't care all that much.

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    $\begingroup$ +1 without any guards no prisoners would ever work, and they would all escape by sundown (or as long as it takes to tunnel to freedom using their "work digging tools" in case they're underground) $\endgroup$ – Xen2050 Oct 2 '18 at 7:51
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    $\begingroup$ ...if they don't starve to death first $\endgroup$ – nzaman Oct 2 '18 at 12:00
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    $\begingroup$ +1, there must be at least one person running the camp, otherwise how do you feed the inmates, deal with medical problems, etc.? Surely there's a soldier or two who can't work on the castle due to sickness or injury. Just get that guy to do the releasing job too. $\endgroup$ – K. Morgan Oct 2 '18 at 15:21
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A particularly tight turnstile is the one person door you're looking for.

The coin verification system is pretty much the only thing that would be difficult for medieval technology, everything else is fairly simple: secure sections that can fit one person at a time, and gears that allow only enough rotation for one person to go through when the coin is inserted.

If it's good enough for amusement parks, it's good enough for prison.

Image of vertical turnstile

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  • $\begingroup$ In particular, you should be using a full height turnstile and ensure there is inward facing spikes or barbed wire over the top to prevent people from trying to climb over. The rest of the area should be fenced or walled off with a similar approach. $\endgroup$ – Shadowzee Oct 2 '18 at 2:23
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    $\begingroup$ A coin verification system can be built around clockworks - vending machines used mechanical coin sorters until the 1980's at least. Medieval clockworks were expensive, but robust. This clockwork monk is far more complex than a coin sorter: io9.gizmodo.com/5956937/… $\endgroup$ – pojo-guy Oct 2 '18 at 2:46
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    $\begingroup$ "If it's good enough for amusement parks, it's good enough for prison." A truism if ever I've heard one. $\endgroup$ – Jeremy Oct 2 '18 at 10:28
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    $\begingroup$ it's worth noting that usually these are installed on exits and rotate only one way - the fixed horizontal bars prevent access through the gate in the wrong direction. That said - I've crawled under these in the past. (a token operated one where I dropped my token which rolled through the gate - I went back through and deposited the token to gain entry again - well the token was worthless elsewhere, so no point in keeping it) $\endgroup$ – Baldrickk Oct 2 '18 at 15:54
  • $\begingroup$ @Baldrickk they're common at entrances to sports grounds too $\endgroup$ – Chris H Oct 4 '18 at 15:50
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Another option is a kissing gate. These are designed so that you push the gate open, which is only a small travel distance. Then, there is a small area to stand. The gate closes behind you and you can walk out. Depending on the size of the standing area, only one person could fit. There is one at the University of Oxford that is like this.

Image from Google Image search

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  • $\begingroup$ What keeps you from holding the gate slightly open while walking past it to get out? $\endgroup$ – Kat Oct 2 '18 at 15:59
  • $\begingroup$ You can keep it from touching the other side as you walk out, but the space left is not wide enough for a person to get through. Think of the total width of the passage (travel distance of the gate) < thickness of 2 humans. $\endgroup$ – shlady Oct 2 '18 at 16:38
  • $\begingroup$ But if you kept it from latching shut, then someone can tailgate after you walk past it, right? $\endgroup$ – Kat Oct 2 '18 at 23:33
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    $\begingroup$ Coincidentally I once had to climb over that exact gate when I found myself in that park after closing time. Those spikes aren't much of a deterrent if you're reasonably agile. $\endgroup$ – jymbob Oct 4 '18 at 15:23
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    $\begingroup$ @jymbob - Presumably Oxford thought razor wire wouldn't be a good look. I'm guessing an Evil Overlord would feel the opposite. $\endgroup$ – T.E.D. Oct 5 '18 at 12:09
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Water Clock as an airlock

The exit room is part of a water clock. The room is filled with water once per hour, and flushed every 12 hours. Inside the room is a seat with a helmet. If you have placed your coin(s) in the slot, the helmet descends upon whoever is in the seat and provides air for them for the next 12 hours. The floor drops (including the seat) to flush out the water. When the water is flushed out, so is the prisoner. Either they are flushed out dead because they didn't pay properly, or they are flushed out alive to freedom.

Bern Water Clock

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    $\begingroup$ I think suffocation is the only real solution. I looked at all these answers with the perspective of carrying a kid on your shoulders/back and most of them failed. If you submerged someone, and made the air-source un-removable—like a latch or kill switch—then you can't share the air and only one person can survive. $\endgroup$ – Nathan Goings Oct 2 '18 at 15:21
  • $\begingroup$ An air helmet? With medieval technology? Does it just have a really long bamboo straw or something? $\endgroup$ – Pharap Oct 3 '18 at 13:21
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    $\begingroup$ @Pharap A leather liner inside a metal cap, where the leather is sealed around the neck with a drawstring would allow air inside the helmet and the helmet to be sealed otherwise. A leather tube with a coiled wire inside would provide the air to be flowing through (both one in and one out). The token could trigger whether air or water flows through this tube. $\endgroup$ – Keeta Oct 3 '18 at 13:28
  • $\begingroup$ you would need a guard around once per hour. prison workers can't be trusted to do the job reliably :) $\endgroup$ – theRiley Oct 3 '18 at 14:34
  • $\begingroup$ @theRiley What job? The clock just runs from water flow. There is no need for any interaction other than the person attempting to leave. $\endgroup$ – Keeta Oct 3 '18 at 14:52
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One neck only

Although two thin people can try to squeeze into the same space, it would be essentially impossible for two necks to fit in a hole large enough for one neck. This is basically because human bodies are flat, but necks are round. The information I could find indicates that the vast majority of necks, from age 11 up, are between 10 and 16 cm in diameter. I reckon a 16cm neck could be squashed into a 14cm hole without causing strangulation, but two 10cm necks couldn't.

How do we utilise this? Well, we have a long underwater passage sealed at the top with a wooden cover. The passage is too long for a person to swim through without drowning. It is also very narrow so swimming would be quite awkward. However, the cover has a single neck-hole. One person can have his/her body submerged in the water and his/her head above water. Here is a simple way to implement this idea: (not to scale; the passage is too short)

The circle would need to be locked so that the cutout could not progress beyond the exit pool unless enough coins were provided, then it could be rotated to the entry pool. One person could enter the pool and place their neck in the cutout, then start edging sideways around the perimeter of the circle. Eventually the person would be able to pass their head through a cutout in the wall, much too small for a person above ground to squeeze through. Finally they would arrive at the exit pool, which would re-engage the locking mechanism, and the person would get out of the water.

The neck cutout would be too small for the person to pull their head beneath the water, thus preventing two people from sharing air during the journey.

Perhaps it would take an hour or two to push the heavy wooden circle all the way around, but since it floats on water, it is plausible for one person to be able to rotate it.

The wall does not need to be straight as depicted; for instance, it can surround only a small area within the "prison" side, which would be useful if a prisoner might need to quickly get out without being exposed to attacks by envious prisoners. It would still be a long way to the exit pool so someone trying to swim would not make it.

This doesn't have a built-in way of purging the dead. However, the floor could be made into a ledge, so any who drown would sink to a deeper area below. That said, whenever a prisoner leaves, the circle would rotate for a really long time so prisoners would realise they have no chance of holding their breath long enough.

This system is nice because apart from coin handling it doesn't have to somehow move and dispense items (like reusable keys or helmets) nor does it need any "active" process like filling or draining water. The locking mechanism would only need pegs, guides and simple latches.

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  • $\begingroup$ this is brilliant. $\endgroup$ – zcaudate Oct 7 '18 at 10:07
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    $\begingroup$ I love this answer. That being said I think it would be possible to circumvent this with a tube, as you could fit a breathing tube along with the first prisoners neck. Then the second escapee would be able to breath through the tube and follow the first one who has their neck sticking out of the whole. Tubes could be made from something like leather. $\endgroup$ – trallgorm Nov 20 '18 at 21:13
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A Prisoner Guards The Door

A highly trusted and experienced prisoner is paid double the normal rate of freedom tokens to guard the door. The caveat is that if headcounts at the end of the day do not match the release manifest he will be severely punished and lose whatever freedom tokens he has accumulated. The door is a regular set of doors that are rigged via an iron bar so that only one may open at a time. The 2nd door is locked, and if the prisoner-guard is not presented the adequate number of tokens he will not open it and the offending person will lose all of their tokens. If two people try to leave they will both be credited with a negative balance equal to the amount it would take to buy freedom.

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    $\begingroup$ The method of having select prisoners guard their peers has been used successfully by prisons and POW camps for a very long time. This practice was one of the reasons for Milgram's Obedience Experiment. $\endgroup$ – Andrew Brēza Oct 2 '18 at 23:09
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    $\begingroup$ Hell, we STILL use it in an unofficial capacity. In prisons or jails "pod bosses" are afforded extra leeway and privledges if they agree to use thier power to maintain order and stability in thier prison or jail pod or block. Its an unspoken agreement between the guards and whatever inmate happens to be running things on the block. $\endgroup$ – TCAT117 Oct 3 '18 at 1:47
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    $\begingroup$ if you use a prisoner as a guard, you've just manufactured another guard. which goes against the fundamental problem requirement. $\endgroup$ – theRiley Oct 3 '18 at 14:51
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    $\begingroup$ nope. The problem is that he doesnt want to waste money HIRING a guard. You dont have to hire a guard if the guard is a slave paid in contrived non-monetary value tokens. $\endgroup$ – TCAT117 Oct 3 '18 at 19:49
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    $\begingroup$ You would still need to have someone trusted to verify counts at the end of the day, though. Which is not really any different than hiring one guard for your single exit door. $\endgroup$ – Miral Oct 9 '18 at 6:33
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Low air cavity

Think about this. The prisoner uses his money to open the door, goes through a corridor or whatever, until another door closes behind him. The point is that he finally finds himself in a closed room with little space. Even worse, the room starts getting filled with water. He probably thinks he's going to die. But the water doesn't reach the ceiling, leaving enough space for him to breath. At least while there's enough air (More people will consume more oxygen).

After some time, the exit door opens and the water is evacuated. If whoever is there is still alive, he's free.

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    $\begingroup$ different size people consume varying amounts of air over time - but it's an interesting concept. $\endgroup$ – theRiley Oct 3 '18 at 14:33
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    $\begingroup$ Also if they panic and start hyperventilating, but not as much as 2 people. And even with that possibility, the dictator needs to ask himself "do I really care if they nearly die? Or if some of them end up dying?". $\endgroup$ – scrp Oct 3 '18 at 15:12
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After spending tokens, the prisoner may pass through a revolving door. Let's say at most two prisoners make it through. Next, there is a jump from one ledge to another (and a gruesome pit beneath). The ledge to land on is a switch that automatically spins the other ledge by 180 degrees. As soon as the first person lands, the second person gets flushed into the pit.

This principle can be extended in case you believe 3 could make it through the revolving door and one may be able to throw the other two onto the ledge instead of one after the other trying to jump. After landing on the ledge (and flushing the third into the pit), the two face another ledge (or another problem of a similar structure). In other words, you don't need a barrier that lets exactly one prisoner out, you only need a sequence of problems that are more difficult the more prisoners try to jointly achieve it.

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  • $\begingroup$ this is a decent and fairly low-tech answer. well done!. probably could do it in one stage by making the opposite ledge only big enough for one set of feet, and far enough that you couldn't carry someone with you on the jump reliably. or make them go back and forth a few times between the ledges, as you were alluding to in terms of a sequence of challenges. but - i dont see this is a guarantee - a spectacularly strong & athletic person might carry a buddy through. some other filter would probably be needed to drive it to certainty. you also need a power source for your rotating ledges ;) $\endgroup$ – theRiley Oct 3 '18 at 14:43
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Ignoring that the dictator really can afford one person to guard the door, and might want to have some sort of celebration when somebody actually gets out and require them to wait for the celebration....

Your original problem was that sometimes one person is as big as two others, or as heavy as two others, etc. So most of the ways you could measure whether it's one person or two would fail in the extreme case.

The easiest thing is to just ignore the problem. Occasionally let two tiny people get out in place of one. Or a great big person can't get out and let him complain about that.

But failing that, with medieval technology, have the exit through a moat, a water-filled corridor. Above There's an air gap at the top that a person can breathe through. To get out, he puts his neck in a stock that leaves his head in air and the rest of his body in the water. The only thing that can stick into the air is his head. Anybody who tries to swim through gets no air, and anybody who tries to crawl through the air space finds a hole at the end just big enough for a head. Two people, no matter how small, cannot fit both their necks in the stock. Arrange that it takes long enough to get through that somebody who tried to hold their breath and swim through the bottom must drown.

I think this could be done with medieval technology. The device to make it take a long time to get through might be tricky.

Would it work? Probably not. People are tricky, and if you leve them unsupervised they will think of something. A snorkel that can stick through the stock past somebody's neck. Etc.

If you make any part of it out of wood, people will find a way to saw it, cut it, or burn it. If it's metal bolted together, they will find a way to get pliers or wrenches. If the water is above the water table, they will find a way to siphon it out.

Anything one engineer can make, another engineer can unmake given sufficient unsupervised time. But it's fun to try to think of ways.

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The door leads to a long corridor full of poisonous gas; the token is an antidote tablet.

Not quite sure if it would be feasible with medieval technology, but you do have some artistic license. Let the gas be slightly corrosive so that any improvised gas mask or other type of protection will be inactive.

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They have security doors.

You have a hall with a door at both ends.

First they have to pay in their token(s).

When the first person enters the door closes behind them. After some kind of check, say a weight check, and after door 1 closes and locks then door 2 opens.

You can even combine this with a turnstile before door 1 to prevent 2 people from going through door 1 at the same time. Make turning the turnstile hard so they have to push it hard to turn it, this would make it even harder for 2 people to reach door 1 at the same time.

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    $\begingroup$ And if there is a kid ridding piggyback of another kid, or a light adult ? How do you make sure the doors are really locked and not stuck ? $\endgroup$ – Don Pablo Oct 2 '18 at 7:58
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Your freedom costs your weight in gold!

This way, you can use your weighing system and be sure of the correct result. If people want to pool their resources then they'll still only be able to let out skinny prisoners who, as other answers have stated, have probably been working harder than the larger prisoners.

It makes it unfair on those with glandular conditions and whatnot but this is a medieval prison, fairness be damned!

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The 'door' is a submerged tunnel. Across the tunnel are air pockets that have only enough space to fit one person's head. Along the way there are turnstile doors that need a key(given from the automated machine) to open so the key cannot travel back down the tunnel.

*Since swimming was a rare skill a rope to pull yourself along would work just as well.

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  • $\begingroup$ I had a similar idea whereby the initial token unlocks some kind of pipe/reed that can be used as a snorkel, the prisoner has to travel through a room filled with water with a slot cut into the roof that's wide enough for the snorkel but doesn't allow enough space for someone to put their head above water and breath. The same problem applies to both, there's nothing to stop two people sharing the snorkel, or two people taking it in turns to pop their head into the airspace, take a lung-full and then make way for their accomplice. $\endgroup$ – delinear Oct 2 '18 at 11:12
  • $\begingroup$ @delinear, I also thought of a pipe/reed. I honestly think suffocation is the only true solution. Simply make the pipe/reed a one time connection. That is, it can only be used by one person, if they try to share it, it fails and they both die. $\endgroup$ – Nathan Goings Oct 2 '18 at 15:23
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    $\begingroup$ you'd need a guard to reset the keys $\endgroup$ – theRiley Oct 3 '18 at 14:45
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While this is a weird set-up, a single person "door" is possible.

Imagine a see-saw filling a hallway. The pivot point is on the middle of the hall, and looking at it from the starting point it appears to be a ramp. You walk up the ramp for a bit, and then it starts to level off. Walk past the pivot and it descends to ground level, and you walk out the exit.

If your buddy tries to run on the ramp behind you, the combined weight keeps the ramp locked in the "up" position. Once you are off the ramp, it can either be reset by a flunky, or the ramp is weighted enough on the "entry" end to swing back to the "up" position. The technology isn't difficult, an inverted roof truss arrangement works, much like this car ramp system:

enter image description here

One end is down to make a ramp, and in this case the other end is braced so the car remains level once the car rolls forward enough to tilt the ramp. Remove the brace and the car can roll forward and off

enter image description here

A different variation of the same idea

A few refinements could be to have the entry behind a door or right angle so only one person at a time can actually approach the ramp. So there is a potential one way door without too much high tech.

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  • $\begingroup$ Definitely possible with medieval tech, and seems like it would work well! $\endgroup$ – Redwolf Programs Oct 2 '18 at 1:22
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    $\begingroup$ that just makes the problem recursive the ramp only works if ony one person can approach it, how do you make sure only one person approaches it? $\endgroup$ – John Oct 2 '18 at 1:51
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    $\begingroup$ What John said is right... if two people hug each other and walk up the ramp, they would trick it into thinking there is one person. It works with a car, because a car is going to be long enough that you can both block the entrance while only allowing one, but with people, your ramp would either be too short to block a person, or long enough to allow multiple people to stand on it. The follow up refinement comment is just the same problem in a different location. $\endgroup$ – Shadowzee Oct 2 '18 at 2:13
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Option 1: a turnstile

I've worked in a few places where they had these:

A regular full-height turnstile

You can't really fit two people, but I guess desperate people might try it anyway. To counter that, the solution is to make it tighter. I'd assume those who have carried enough big rocks to earn freedom would be rather lean, so all you need is a turnstile where an average human can barely fit. Plus-size prisoners be damned, they clearly haven't done enough work. If you already have the medieval token validating machine, this is trivial.

Option 2: part-time guard duty

Sure you can't afford a full time guard for some reason. But then you could just decide people can only get out Saturdays from 9 to 10. They've waited this long for freedom, they can wait for next Saturday. Like the poet says, everybody's working for the weekend. Of course, there are problems to this, the possibility of bribery, letting someone out by mistake, or, worst, union strike.

Option 2, part 2: part-time you

Like the French say, you're never better served than by yourself. You are immune to bribery and mistakes, and besides you do want them to know you let them out because you're such a generous and nice guy. Never neglect the effect of the personal touch.

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  • $\begingroup$ "everybody's working for the weekend." Pretty sure that's talking about working so you have enough money to have fun on weekends. Not actually working ON the weekend. $\endgroup$ – Aethenosity Oct 2 '18 at 22:07
  • $\begingroup$ @Aethenosity What I meant was it's working so you can get out and have your freedom, so it totally applies. $\endgroup$ – AmiralPatate Oct 3 '18 at 5:50
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A saddle riding on a track that twists and turns like a bucking horse will force the person to hold on with both hands and any other person will have their center of gravity off center, and that would make the bucking movement very hard to counter, throwing the other person or both of them off the saddle into the try-again pillows (concrete).

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    $\begingroup$ This could be overcome with skill, luck, or two people tied together. $\endgroup$ – Nathan Goings Oct 2 '18 at 15:24
  • $\begingroup$ @NathanGoings then anything could be overcome with skill, luck or two people tied together, except for heartbeat scanners that have lasers that eliminates all but one beating heart. (then again it could be thwarted by drugs that slows your heart so you could appear dead) $\endgroup$ – workoverflow Oct 3 '18 at 6:47
  • $\begingroup$ not totally foolproof, but a cool idea, and almost foolproof. love those try-again pillows ;) $\endgroup$ – theRiley Oct 3 '18 at 14:58
  • $\begingroup$ @workoverflow, I think the water/suffocation related answers would be hard to beat considering the constraints to the original question. It's similar to your heart argument but lungs and more realistic. $\endgroup$ – Nathan Goings Oct 3 '18 at 20:24
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How about a very long sluice / underwater tunnel?

You simply then make only one diving helmet available (and weights for feet etc), with a deadly ceiling (venomous spikes?). This way you can't have a floaty hanger-on (the spikes!); the helmet is typically so heavy you can't swap mid-swim (and would live on 50% of the expected oxygen while doing more effort due to costume swapping) --- taking helmet off you'd float up, putting it on underwater you'd plummet down out of your mate's reach. The pressure differences would get you, if you're not simply following the underwater-staircase... and it's dark in there too, no communication possible.

Combine that with a turnstyle (that looks like it will only let one person pass, but inevitably people will try two and succeed at some point) so there's no information feeding back to your prisoners (and don't re-imprison any successful leaver!). You'll need occasional cleanup to display the dead bodies of failed escapees.

Actually that may be the most straightforward solution: Turnstile followed by a bendy route-out-of-sight. Employ for a limited time guards that kill both leavers, then 48hrs later drop the dead bodies in the main prison... this will raise enough questions and make them stop trying: How!? What?!
Without this rule, when one has enough money to escape, then a cheapskate asks "can I tag along?"; you say "yea, gimme all your gold; but I take care of myself if in doubt out there" --- but in this situation it's "I'm paying full price I'm not taking any risk I go alone". I think it's cheaper to employ guards for a week at the begin than to construct elaborate defences... and maintaining defences is expensive too. YOu'll want a periodic headcount to see if people are escaping and you need to shore up defences/ deploy 'project fear' to stop them from attempting. [Maintenance costs may depend on climate too...]

The prisoners have no idea if the exit is guarded at all (if there's actually two turnstyles, otherwise the legitimate leaver might report back. You'll need some communication barrier otherwise the successful leaver will send messages back in with balloons, skywriters, carrier pigeons, rocks-with-messages, fireworks/signal arrows, ... .

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The Centrifugal Sally Port Singleton Sortation System Solution

The system comes with a complete set of instructions posted both inside & outside the sally port, complete with clear illustrations, explaining the tasks required to complete the emancipation process.

Prisoners attempting the undertake the emancipation process do so entirely at their own risk.

The first sortation is by aptitude & a certain minimal level of mechanical competency. Those prisoners not capable of following instructions will have to adapt themselves to prison life indefinitely, or assume the various risks as described below.

The interior chamber features a centrifugal sortation mechanism. The interior door opens for a fixed interval upon inserted coin weight exceeding the price counter-weight, this easily modified as the price of freedom fluctuates with market forces. The prisoner enters.

The system is in its first operating state. It is driven by a mechanical stepper actuated by sisal twine connecting linkages. As each step is completed, actuators or timers trigger the stepper state advance step wise, until the emancipation cycle is completed, the door opening for a fixed interval, during which the freed prisoner exits, then closes. Each step has a timer-driven delay which, if exceeded, resets the chamber to its initial state, the interior door opens and the prisoner, bereft of his coin, returns to from whence he came, likely motivated in future to read the instructions more carefully.

First the prisoner ready to be freed, once inside, must wind all the timers, then charge all the springs, in the following order: carousel brake engage spring, inner then outer door spring assemblies, the centrifugal shaft main coil spring, hand and feet irons compression springs.

The irons are all formed to fit the normal range of adult human limbs - prisoners missing more than a single member, or having deformed or in any manner terribly oddly shaped limbs get extra rations, no freedom ever. All are briefed about the situation in their orientation session the first day, so no unhappy surprises later.

As soon as all timers have been wound & each spring is charged, the stepper advances, opening access to the carousel. The prisoner steps onto the carousel near the main shaft where there are positioned hand and feet irons.

Feet and hands are placed into the irons carefully one by one, feet followed by hands. In each, a contact button fires the compression spring, placing a good deal of tension upon each limb. Circulation may be inhibited, but only for a short while.

When the last spring is fired, the stepper advances, the shaft brake release latch is triggered, the carousel around the main shaft, tensioned by the pre-wound shaft coil spring, then starts rotating, rapidly gaining a rotational velocity sufficient to fire any hangers on or in any significant way improperly tethered prisoner into a wall studded with 12" sharpened spikes anchored upon 3" centers (the chamber is cleaned out nightly, or as found to be necessary). This is the main sortation phase.

After an interval sufficient to effect any necessary sortations, the stepper thereupon advances, firing the carousel brake engage. The brake arrests the carousel rotation moderately quickly. After a delay sufficient to ensure the shaft is stopped and vertigo has subsided, the stepper advances, the irons compression springs are unlatched, and the prisoner may disengage themselves from the irons, turn around, now prepared for the final exit sequence.

Being careful to avoid the aforementioned spikes, the prisoner then pushes the clearly marked external door release button, and is on his way. After the final delay, as previously described at the outset, the external door closes, the stepper resets to its initial state, the chamber now ready to service the next emancipation cycle.

If near London, please stop by our dimly lit, poorly ventilated & extremely drafty workshop. We forge, monger & mend a full line of dungeon hardware.

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    $\begingroup$ Huh...pretty cool! $\endgroup$ – Redwolf Programs Oct 3 '18 at 14:51
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    $\begingroup$ Welcome to Worldbuilding! I think this answer could benefit from an explanation of why the mechanism described meets the question criteria. $\endgroup$ – Jared K Oct 3 '18 at 14:53
  • $\begingroup$ @Jared K - you are probably right on the timers,this is more late renaissance tech. the springs i think are remotely realistic. the main thing i wanted to tackle was a repeatable system not requiring guard intervention (beyond daily cleaning) guaranteeing singletons. let's substitute glass timers contour-weighted to invert on empty, landing on actuators :) $\endgroup$ – theRiley Oct 3 '18 at 15:09
  • $\begingroup$ just thought of another potential problem. two inmates ruggedly sown into a single garment would cheat the system. So I will stipulate that any sort of even potential sewing contraband found during periodic cell searches will be met with slow, painful death -or- all inmates are kept naked, kept well clear of any cloths or fabrics or hides or fibers of any sort whatsoever. $\endgroup$ – theRiley Oct 3 '18 at 17:14
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If he doesn't want to devote any of his soldiers to the task, another prisoner could do it instead. This prisoner would be tasked with monitoring the gate, and ensure that only one person is released. This is how he/she would earn his coins.

To prevent people from forcing their way through once the door is opened, the easiest solution would be to have an outer and inner door. The outer door cannot be opened until the inner door is closed, and the inner door won't be closed if more than one person has attempted to enter.

To discourage the gatekeeper prisoner from cheating and deliberately allowing more than one person through, the gatekeeper prisoners could be kept isolated from the other prisoners at all times. They could also be told that soldiers guard the outer door.

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    $\begingroup$ If I as a prisoner control the gate, I don't need no stinkin' coins - I get a free pass. And I'm pretty likely to let everyone else out too since there are no guards there to prevent it. Even if the prisoner-gaurds are in a separate compound, there must be a way to get to the gate if my own coins are ever to do any good. $\endgroup$ – brichins Oct 2 '18 at 16:49
  • $\begingroup$ @brichins it might work if the prisoners believe there are guards outside the gate. Even if there really aren't, they'd have no way to verify this. $\endgroup$ – user1751825 Oct 2 '18 at 21:21
  • $\begingroup$ @brichins or, you have 2 completely isolated groups of prisoners. Each group operates the gate for the other, and each are told that they cannot get their coins or eventual release unless they follow the rules. $\endgroup$ – user1751825 Oct 2 '18 at 21:25
  • $\begingroup$ Two groups is basically the opposite of the prisoner's dilemma - the worst possible personal outcome for group A to not help group B. If you have guards, they're the actual control system; if you have no guards, prisoners have no reason to not just open each other's gate and let everyone walk out. $\endgroup$ – brichins Oct 2 '18 at 22:20
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    $\begingroup$ creating guards avoids the problem. $\endgroup$ – theRiley Oct 3 '18 at 14:56
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Let's get a couple of things out of the way. . 1. any door that works for a large person, will work for 2 smaller people if they squeeze into a fat man's clothing.
2. Without human, all mechanical defenses can be defeated. Let's ignore that.

The only way to make this work is to give all your prisoners ankle bracelets. If a bracelet is ever removed, they explode, or the army charges in and executes the violator.

Next you have a terminal. If you earn enough coins, you can go in, put in the coins and type in your prisoner number. This deactivates the bracelet.

Next you have a 100 foot hallway with 2 airlock doors. Both doors can never be open together. If there is anyone in the hallway with an active bracelet, the second door cannot open.

So when prisoners who are freed go in, they can open the first door. Close it behind then, then walk over and open the second door. If anyone with an active bracelet gets into the hallway, it can't be opened.

Your bigger problem may be people going the other way as they don't have bracelets and could travel back and forth freely. A simple 1 way revolving door with crossing bars as a filter like in the subways solves this.

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  • $\begingroup$ Since this is supposed to be medieval tech, you would want clockwork dials or something, probably not a keypad. A bracelet that explodes if removed sounds very tricky to make reliable. But I can't think of any tech even close to medieval that would detect whether a bracelet is within a region or not. $\endgroup$ – aschepler Oct 8 '18 at 14:30
  • $\begingroup$ There would be no way to make anything like an exploding ankle bracelet using mediaeval technology. Even ignoring the complication of detecting proximity. At best they may have had rudimentary knowledge of black powder, but that would be far too bulky. Also the only way they could have ignited powder would be to use a flame. A prisoner could defeat this simply by soaking the powder. $\endgroup$ – user1751825 Oct 8 '18 at 22:32
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Once a day (or once a week if manpower is that stretched)

Reinforced "airlock" cages. Each cage is large enough to moderately comfortably hold one person (has a chair included) and can be accessed by appropriate tokenage. Once a day at a designated time a guard comes, overlooks the cages, and assuming there is only one person per cage locks the entrance sides, then unlocks the exit sides and all the liberated prisoners are free to go. This requires only a small amount of effort on the guard's part (maybe 5 minutes?) while still ensure only appropriately paid prisoners are let out.

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We have several answers that touch on air supply but I don't see that they are adequate--either the device could be shared or how do you calibrate it?

Thus:

You put your tokens in the slot and then put your head into a cavity in the ceiling that air is slowly being pumped into--enough to sustain a person. There is a spill valve on the top of the cavity that will dump the air, this value is normally open. To avoid losing the air it must be pressed (not a lot of pressure is needed) closed--and given the nature of the device the only available means of doing this is with the top of one's head.

If two people try to cheat when they swap all the air in the cavity is spilled out. While there is enough air being supplied even for one large person (which might suffice for two small people) there isn't enough for two given that you keep losing it every time they change places.

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In medieval tech, perhaps there are vicious poisonous/deadly thingies, with known antivenom/prophylaxis sources. These don't have to be snakes, perhaps there are lethal mosquito-bourne parasites (a lethal version of malaria/ebola?) and some local plant or product that's been found to confer immunity or a cure, when eaten.

Your token gets you enough antivenom/prophylactic substance/antidote for one person. The exit room is designed to lethally affect anyone entering it. Problem solved.

(On an overview, you're looking for things that are definitely able to be rationed one per person. Air is an obvious solution, breathing gear too. Really, anything that you need a certain amount of to live, and a situation where two people trying to share the resource will die, will answer the question. But I prefer toxins to air/water. So pretty, those blue agonised deathly faces, when you dump them back in among the prisoners with the tag attached "another person who thought they could beat the exit token system" ;-) )

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  • $\begingroup$ I was about to write this exact answer. Here are two obvious issues people might think of and why they are not really issues: 1. poison prevention - prisoners will try to shield themselves from the poison. Fortunately there are many kinds of poison, many of which require more resources and knowledge than medieval prisoners have to shield from. 2. Diluting/sharing the antidote - I am no toxicologist but I think antidotes quantities work based off weight. Trying this hurts the chances of the other plus any failed attempts are immediate death, again they are ill informed, medieval prisoners $\endgroup$ – Jesse Oct 5 '18 at 5:56
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This is a problem of supply and accounting rather than technology and doors.

Each prisoner's stones needs to be collected and brought to wherever you need them. During this collection his stones are counted. This can be done by giving each prisoner their own section with a name or code associated with it, and dogtags that have that name or code on them. Or if you want to risk people thinking it's a concentration camp regardless of how well treated the prisoners are, give them a tatoo so the dogtags can't be stolen. You could combine it: Dogtags have a name and a symbol on it, as does the section. The prisoner gets a symbol tatoo and a dogtag so people can't steal dogtags and collect more coins.

After hauling the stones to his section, the prisoner is tasked with stacking them in a particular way that allows the collectors who come with carts to do an easy count. Then the prisoners themselves are tasked with helping load the cart with the stones, once finished and no foul play is found they will get their coins based on the amount of stones found in a pile. This means that it might take a few weeks for people to get their coins as transportation might not come every day for their particular pile, but they will get them.

If you have limited space, for example because the stones are hauled to a warehouse, the prisoners are put into groups with dogtags. The group as a whole brings stones and puts it in their section all stacked and ready for transport. Once on the carriages the group gets paid with everyone an equal share. This promotes self-regulation of the group, when one slacks off the group will be punished with slower coin gains. Should the slacking off the too much the group might punish that person by stealing a portion (or all) of their coins after collection. Groups also help self-regulate stone stealing. For example an enterprising person who decides that hauling stones so far isn't worth it if he can steal from a pile nearby. Groups can police their own section, and with proper labor division there's always someone stacking stones and guarding it from intruders while the rest is hauling the stones.

Once enough coins are collected, the prisoner can pay the guys who give the tokens in the first place (easy recycling).

The beauty is that this system uses the simple things you'll be needing anyways: people who collect and distribute the work (not just stones as mentioned) and who can assertain how much work was done upon collection and give out the reward. If these people are corrupt then the collection and transportation of the work would already fail and theres no worrying about prisoners getting loose too quickly, and those people would be more inclined to keep their prisoner labor high than let these people out.

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