In my story, the princess escapes her prison through the window. The window has a lock on it, made by a legendary locksmith, that's said to be unpickable. But either due to the locksmith not being told what the lock was for, and/or the lock being installed by the lowest bidder from a nearby town, it can open easily from the outside.

The princess knows this, and she does just that after removing a small glass pane that was loose due to wear and tear of the frames. Breaking the window would be faster, of course, but also more likely to attract attention.

My main question is about the bit in italics. Is it conceivable that a lock such as this (one-sided) could exist, using medieval / early Renaissance technology? (It doesn't have to be common; the work of a mad genius is good enough.) Is it conceivable that the one-sidedness could be a side-effect brought about by faulty installation? I tried looking for 'one-sided locks' but this mostly results in plain deadbolts, which...is technically correct, but not what I'm looking for. Ideally those windows should still open from the inside with a key (e.g. to wash them). I'm interested in locks that can be opened automatically from the opposite side to that in which they are locked. Sort of like some hotel room locks, where the door unlocks automatically when walking out of the door, but it requires a key the other way around.

(If this is not doable, I'll probably go with 'actually, the window was never locked all along', but I'm curious if this can work out! I know nothing about the history of locks.)

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    $\begingroup$ most medieval door locks were one sided, prison doors rarely had locks instead they just had latches that the prisoner could not reach. And remember the best lock in the world can't secure a door built by an idiot. youtube.com/watch?v=4YYvBLAF4T8 $\endgroup$
    – John
    Commented Sep 25, 2022 at 15:04
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    $\begingroup$ It's your story, it's up to you. If a Hobbit can defeat Saruman... $\endgroup$
    – L.Dutch
    Commented Sep 25, 2022 at 15:36
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    $\begingroup$ (1) There was nothing even remotely like an "unpickable" lock in the Renaissance and the Early Modern period. The first lock which posed at least half-serious problems to lockpickers was the double-acting pin-tumbler lock (a.k.a. "Yale" lock) invented in the 19th century. (2) It is trivially easy to make a locking which requires a key from one side but does not require a key from the other side. In some countries, this is the normal arrangement (sometimes even required by the code) for appartment doors and hotel room doors. $\endgroup$
    – AlexP
    Commented Sep 25, 2022 at 16:44
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    $\begingroup$ Glass window and "security" don't match. "Medieval" and "glass window in a prison" don't match. $\endgroup$
    – JRE
    Commented Sep 26, 2022 at 6:59
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    $\begingroup$ @JRE: Having glass windows make sense if, for political reasons, the prison needs to be a gilded cage. For example, if the princess is an action hero but is being treated as a damsel. $\endgroup$
    – Brian
    Commented Sep 26, 2022 at 16:28

9 Answers 9


Frame (ahem) challenge.

The prison's walls are very thick. The handyman who has to install the windows also does the cleaning, once every two months, and he can see that it would be a right pain to get to the glass, all the way down the deep recess. So he has a bright idea: he'll just mount the window on the inside, flush with the inner wall. Easy to get to, easy to clean, just a wipe with a wet cloth.

The princess simply lifts the window off its hinges, which are on the cell side of the wall, and sets the pair of windowpanes aside, still locked with their very expensive, unpickable lock.

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    $\begingroup$ IDK man, I just work here. - Fixing other people's mistakes is my job description. I'll often jest that w/e it is, it's the drywaller's problem, except that's next for me after the electrical and plumbing, so I'll actually do a good job on the framing. Hinges have to go on the inside or someone outside can get in. Which is why I begin every job with a question: Objective? W/o any specifics, I'll assume you want to save money, not imprison people, +1. $\endgroup$
    – Mazura
    Commented Sep 28, 2022 at 2:15
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    $\begingroup$ Easy mistake if the lowest bidder is not told this room is a prison. $\endgroup$ Commented Sep 28, 2022 at 4:16

Actually, it is trivial, even with today's technology. Most door locks are built to lock from one side and open easily from the other. If the locksmith wasn't told about breakable glass, this could be the type of lock you got.

The unbelievable part is "unpickable". That doesn't stop anybody using the term.

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    $\begingroup$ There are entire hobbyist groups that consider that word a challenge and so far they're winning. $\endgroup$ Commented Sep 26, 2022 at 3:41
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    $\begingroup$ This is the Lockpicking Lawyer, and what I have for you today is a princess trapped in a tower... $\endgroup$ Commented Sep 26, 2022 at 12:15
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    $\begingroup$ The princess must lock the door and then unlock it again to prove it was not a fluke. @SebastianLenartowicz $\endgroup$ Commented Sep 26, 2022 at 15:31
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    $\begingroup$ @SebastianLenartowicz "... It often seems that the best locks out there in terms of pick resistance make few, if any, claims about it. This lock [points to a window labeled "unpickable"] is the corollary to that observation. ... Nothing on one ... 2 ... 3 ... 4 ... OK; 5 is binding ..." 1m10s later:" ... and we got this open." $\endgroup$ Commented Sep 27, 2022 at 2:49
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    $\begingroup$ For that matter, a lock with NO KEYHOLE on the inside of the cell would be unpickable from that side... $\endgroup$
    – Zeiss Ikon
    Commented Sep 27, 2022 at 14:40

Yes... and no...

All due respect to David's answer, he's thinking of modern door locks, which didn't become the form we're familiar with today until the early 1800s.

Medieval locks were either padlocks or surface latch locks that would not have had key access from the other side of the door. It's not that a surface latch lock couldn't have a hole drilled in the door to allow a key from the other side, but what would be the point?

It's worth noting that at the beginning of the medieval era most locks required a small hole (not a keyhole) at the back of the lock to mount the tip of the key. This was to provide leverage and stability while moving the latch itself.

So, yes, it's theoretically possible that someone could have created the double-sided (in modern terms, double-cylinder) locks, but is it believable?

No, it's not. Who would put such a lock on a window? Even back in medieval times, the point (literally the whole point) was to keep people out — and yet you're looking for a lock that's trivially opened from the outside. Yes, I get that you're looking to imprison, but the prisoner is the person "on the outside" you're trying to keep out. (Yeah, yeah, yeah, inside out... work with me.)

You're suggesting a level of incompetence that's difficult to believe. It's on the order of something invented by Bloody Stupid Johnson.

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    $\begingroup$ If it is a completely new device just created by the legendary locksmith who only was told "will ever be opened only from one side", then lowest bidder installer might not know that you are supposed to prevent access to the lock body from the other side. $\endgroup$ Commented Sep 26, 2022 at 15:10
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    $\begingroup$ @Revolver_Ocelot Work with me on this one. The so-called legendary locksmith won't create a lock that can be opened from both sides given an order to produce a lock that can only be opened on one side or such a person isn't legendary. Theoretically such a person might leave the back plate off the lock, allowing a hole to be cut from the other side to access it, but why would the legendary smith be that honking stupid (legendary...) or the installer go to that much effort? My point is that it's a whomping stretch to believe this situation. (*continued*) $\endgroup$
    – JBH
    Commented Sep 27, 2022 at 2:50
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    $\begingroup$ ...If a hole was in the window, the lock could be knocked off (no need for the legendary smith to be implicated at all) because someone could stick a bar through the hole and hit it with a hammer. But at that point there's no reason to ask the question at all. The lock could be bolted to the window from the outside, but that's nothing to do with the lock, either. IMO the OP's premise doesn't meet the minimum standard for suspension of disbelief. Now, if it was intentional.... $\endgroup$
    – JBH
    Commented Sep 27, 2022 at 2:52
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    $\begingroup$ Literally as I was reading your answer I was thinking of a half dozen times in various discworld books Vetinari/the Witches/Leonard of Quirm were "imprisoned" but chose to view their circumstances as reversed. They were safe from the world because they were "locked out" of it. $\endgroup$ Commented Sep 27, 2022 at 14:04
  • $\begingroup$ I checked several locks in my appartments which have definite "inside" and "outside". All of them can easily be defeated from inside. Gun safe lock can be removed from safe body if you have access to the inside. Desk drawer lock will give you free access to the bolt if you chip away thin layer of soft material from inside. Door lock can be disassembled from the inside. All of this is because they are intended to protect from "outside" intrusions. If locksmith was not aware that lock should be protected from "inside" attacks, he could leave access to the internals in case repairs are needed. $\endgroup$ Commented Sep 27, 2022 at 14:27

Medieval locks weren't unpickable.

They didn't have the technical knowhow to make especially unpickable locks. It wasn't until the 1700s ish that they started developing that.

Medieval locks you could pretty easily pick by just getting a similar key with different biting, filing off the parts that don't engage the lever, and using it.

What you can do is make a latch. That can't be opened from the inside, and so it's absolutely secure, so long as you don't have a window you can remove.

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    $\begingroup$ But then you wouldn't bother to have the latch. You're not going to send someone climbing the tower to open the window from the outside, so you would just have a window that doesn't open. A lock is logical as a latch is not $\endgroup$
    – Mary
    Commented Sep 26, 2022 at 0:11
  • $\begingroup$ You might, because that means the princess can't escape, in theory, without a ladder and someone on the outside, which is loud and visible. It's very secure. $\endgroup$
    – Nepene Nep
    Commented Sep 26, 2022 at 10:21
  • $\begingroup$ @Mary You're underestimating how lazy/logical people are and to which degree things are purpose-built. It happens all the time that someone just buys whatever's available and makes it work for their purpose. Windows commonly have latches on the inside, so they could conceivably have gotten one of those (or already had one of those in that room) and just turned it around. $\endgroup$
    – NotThatGuy
    Commented Sep 26, 2022 at 11:53
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    $\begingroup$ @NotThatGuy you have forgotten that this is at most Renaissance tech. There are no such things as off the shelf, standardized window panes, partly because there are no such things as standardized windows for them. It will be bespoke. $\endgroup$
    – Mary
    Commented Sep 26, 2022 at 12:22
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    $\begingroup$ Oh, medieval locks were built to withstand lockpicking. There were all kinds of trick locks, all designed to make something bad happen if the lockpicker "misread" the lock: The lock would break, or jam, or emit noxious substances, or whatever. Such locks were expensive though, you wouldn't lock a prison cell with that (also, there were no prison cells usually - a room without doors and a hole in the ceiling was cheaper). $\endgroup$
    – toolforger
    Commented Sep 26, 2022 at 21:37

Some frame challenges

More like "you need the following conditions to have the scenario you describe", not the "this is impossible" challenges.

Economic constraints

There were no affordable locks in medieval times; they were absolute luxury contraptions. They were typically used on the treasure chests of the wealthy and/or powerful.

Even iron wasn't exactly cheap. You don't have prison cells with iron bars and a locked door, that came only with the industrial revolution when forging became mechanised, which started with watermills driving drop hammers - this became reasonably common in Renaissance times, which is usually considered to be after medieval times.
Even then, it is much cheaper to build the prison as a room with a hole in the ceiling. Nothing goes in or out except dangling from a rope that is held by as many sturdy men as necessary for whatever the weight is: prisoners, food, waste. Has the nice side effect that nothing goes in or out except in the presence of guards.

Personnel is cheap

Or, rather, mechanisms are expensive.

In medieval times, payment for a maid could be food, housing, and two dresses per year, and given that food and housing couldn't be taken for granted and fabric for dresses was pretty expensive, this could be a pretty fair deal.
Guards were even more expensive: Their gear contained metal where leather and wood wouldn't do.

So... again we use the hole-in-the-ceiling prison. The prison personnel is much cheaper than a grille door with a - gasp! - lock.

So we must be in a gilded-cage scenario

Maybe the princess is on display, for political reasons.

Or maybe her captor is just mocking her: See, I consider you so weak that you won't escape even from an unsafe prison.

Or she's the kind of prisoner that you can take hostage to keep somebody from rebelling, but that person would surely rebel if you put her into a standard hole-in-the-ceiling prison because such prisons take away first health, then life - you can't really clean them.

However, there's still the question: Why locks? Keeping doors and windows guarded at all times is still much cheaper than a lock.

So... the captor shows off his wealth.
"Look, I can afford to put her in a locked room WITH NO GUARDS."
(Such an approach to guarding an important prisoner would be considered a bad idea, and indeed the story will progress in her escaping... guards would have prevented that.)

You don't put a lock on a window

It's just too expensive.

Medieval windows didn't have glass, they had shutters to keep bad weather out.

Improved windows had a grill, to keep attackers out (or prisoners in). The grill would be set into the stone, no lock.

Expensive windows had oiled parchment. It's not transparent, but it's translucent so you get light into the room.

A possible scenario

It's not the window.
It's part of a mind game: The Evil Overlord is taunting the princess.
Yes, it's an expensive grill door. With an absurdly expensive trick lock.
And the key hanging from a hook... just out of reach.

The shoddy worksmanship could be anywhere:

  • The window has bars, these were replaced because "rusty does not befit a princess", but the replacement stones that the bars are set into were sandstone.
  • The hook is nailed to a wooden piece of furniture. The hook was replaced, and the new nails were slightly less thick than the previous ones. Or maybe the hook has been replaced so many times that the wood became brittle and any nail could be pulled out by hand.
    The jailers know this, but fixing this would cost money (if only to pay a carpenter to do the job properly), so nothing was done, and somehow the princess manages to attach a rope to the hook and pulls it out.

Another scenario (just shoddy, not elaborate locks though)

Not a lock.
Just a rivet closing a neck ring or manacles. The rivet is a pretty soft metal so you can file it open with little effort, but files are something that only smiths own so it's practically inescapable. (I read that thralls in ye olde times in England had "thrall rings" around their necks that made their status visible; that was fictional work though, so I don't know much about how much, if any, this was in practice.)
Anyway: No file near the princess, problem solved.

Except maybe the smith wasn't competent. Or sabotaging the work.
Or the smith couldn't be bothered with doing so low-level work, and sent the apprentice.
Either way, the rivet was fastened just enough to barely hold, and can be moved out by hand given oil.

Probably not an answer to the question, but I kinda like the idea of an apprentice mucking up a perfectly reasonable and simple job, just because the master didn't have the time to properly deal with the task.

  • $\begingroup$ "A soft metal pin that you put through holes and hammer shorter and thicker." Or, in fewer words, a rivet. $\endgroup$
    – Matthew
    Commented Sep 28, 2022 at 19:39
  • $\begingroup$ Ah I didn't know that rivets are a whole class of fasteners, I have been believing only hollow ones are called rivets. But yeah what I describe is a solid rivet, though the technique described here does not rely on the heads to close the connection, just the diameter increase (though the heads obviously do form). $\endgroup$
    – toolforger
    Commented Sep 29, 2022 at 21:48

The window uses a ward lock.

Before the 1800s, there were very few styles of locks and those that existed were pretty easy to beat assuming you had the right tools and knowledge. The two basic styles were pin tumbler locks and warded locks.

Pin tumbler locks were not like modern pin tumblers because they did not have sperate key and driver pins. They simply had pins that had to be lifted, and as long as you had a pick and a tensioning tool, they were really easy to beat compared to a the Yale Pin Tumbler which is the bases of most modern locks, or if you could figure out the spacing and number of pins, it was trivial to fashion a working key because all pins had the same offset. However, the advantage of these locks is that the pins prevented the latch from just being pushed into an open position. So this is not the kind of lock you want for your story,

enter image description here

The second style are warded locks which is where you have a shape that matches the shape of the key. These were also easy to beat because you just needed a "skeleton key" a style of key that may not match the whole pattern of the original, but can fit through at least part of the ward's pattern to turn the lock.

enter image description here

A pick resistant ward lock would be one where the simplest profile that can turn through the ward to hit the latch would be more complex than any skeleton key a person could easily fashion by sheer chance like this.

enter image description here

In general, both styles of lock are very easy to pick with the right tools, but the real question here is not if the lock can be picked, but if it can be picked with tools that the princess can improvise from found objects in her locked room. With this being the real test, a well made ward lock may seem like the better option.

However, unlike pin tumbler locks, these locks don't actually have any binding mechanism to keep the latch in place; so, if you were to access a ward lock from the backside, all you would see is some manner or rotating latch that you could just turn open with your hand, since under normal circumstances, it is only the rotating action of the key that actually opens the latch.


Simplest form of a lock that doesn't need a key on one side, and is unpickable on the other, is a lock that doesn't use a key at all.

A simple knob-operated deadbolt is well within 13th century technology. On the outside of the cell, there's just a simple knob, but with the door opening outward and no strike-close sprung bolt, it's impossible to jimmy from the inside (without very time consuming and obvious destruction, if anyone opens the door -- for instance to deliver meals and change the chamber pot) if there's a stone or metal jamb. On the inside, there's nothing but, at most, a small iron plate, likely held to the lock body by rivets.


I am reminded of a scene from an old "Robin Hood" TV adaptation. Alas I could not find the clip on-line. The heroes are locked in a prison cell, and the hero asks if they have any chance of picking the lock. One of his companions replies along the lines of:

That lock? No way. I know the man who made that lock and he is an expert. We'll be here until midsummer trying to pick it ... however, he couldn't make a decent hinge to save his life. Let me show you.

The actual escape happened off camera so we are left to wonder if the problem with the hinge was in loose pins that could be knocked out, or in the way it attached to the door or the frame so that it could be prised loose, or the in materials used so that they could be deformed sufficiently to free the door.


Here is an example. The way my home door works is: it is held in place when closed by two bars. Turning the handle extends and retracts one bar, and turning the lock extends and retracts the other. The one that is controlled by the handle is pushed out by a spring so that the door can still close while the handle is not pushed down. If you could reach inside the door frame and push on this bar, then you could open the door without the handle (provided it was not locked.)

If your window is held locked by only one bar, which is extended and retracted by the lock, and that one bar is pushed by a spring so that the window can be closed without the key, and the lock was installed poorly such that this bar is accessible from the outside, then you can open the window without turning the lock and your italicized statement is conceivable.


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