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A city was built in a location that's generally difficult to access (floating in the sky, on a remote island, on top of a mountain or something similar). The primary way of accessing it is by teleporting inside. In this world, skilled mages can teleport from anywhere, but only to a previously built "portal", in a similar way fast travel points work in video games. Small objects, such as food crates, can also be teleported to a "portal".

This city contains one such "portal" for all arrivals which functions in a way similar to real-world airports, with heavy security, customs checks etc. A "portal" can also be temporarily closed, which prevents anyone from arriving until it's reopened.

This makes any large-scale invasion impossible, but a devastating smaller scale attack still seems feasible: a group of enemy mages could coordinate to have everyone teleport at the same time and overwhelm the guards stationed near the "portal" before they have a chance to close it, then proceed to steal valuable artifacts / assassin a target / wreck havoc in some other way.

What mechanism (magical or otherwise) could effectively prevent this from happening?

My ideas so far were:

  • Have teleportation require some sort of "key" (analogous to real-world entry visas) that needs to be in possession to make teleporting to this city possible. However, enemy mages could, over time, steal a sufficient number of these and invade. Or, if this "key" is somehow tied to a person (e.g. in their blood), it wouldn't stop a group of former allies who had a change of heart.

  • Move this "portal" outside the city walls. While this makes a direct attack more difficult, it makes it easy for enemy forces to create a blockade and starve the city of vital resources that would come via this only entrance.

Edit: Just to clarify, the idea is not to make the city completely impervious to attacks of any kind, my main goal is to make use of this teleportation mechanic without worrying it introduces new types of attack that wouldn't otherwise exist. Things like infiltrations and insurgencies are still a very real threat.

Edit2: I see a few answers suggest changing the teleportation mechanic in some way. I'm generally open to that (e.g. adding a delay, requiring more elaborate conditions to be met etc), but one thing I don't want to change is the fact you can teleport from anywhere, as the original reason for introducing this mechanic was for the main characters to be able to access the city without having to explain how they managed to get back.

Regarding enemies building their own second "portal" after infiltrating the city: the assumption is that such a venture is possible, but extremely costly in terms of both materials and time and relatively easy to detect.

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    $\begingroup$ limit the number of teleports per unit time. $\endgroup$
    – Allan
    Aug 12 at 1:53
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    $\begingroup$ Maybe a bit off-topic, but what prevents enemy mages from building their own portal in the floating city? $\endgroup$
    – Wolter
    Aug 12 at 11:03
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    $\begingroup$ so you basically want to rebuild sigil, city of doors? a problem with a teleportation-reachable place is that as soon as somebody teleports there, he is already inside... so you'd need only a single weapon of mass destruction, be it a god or a bomb or a sickness etc.... $\endgroup$
    – clockw0rk
    Aug 12 at 15:13
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    $\begingroup$ Sounds like Stargate $\endgroup$
    – DKNguyen
    Aug 12 at 17:16
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    $\begingroup$ @vulcan_ it's true that this adds an entry point, and any entry point can be attacked, but I'd like to minimize a new type of attack specific to this (magical) entry point. I'm perfectly fine with a type of attack that would be equally feasible (or less feasible) if we replace the portal with e.g. a giant non-magical gate, such as attackers pretending to be tourists, concealing a weapon from the guards, then proceeding to wreck havoc once inside. This type of attack isn't caused by teleportation and thus would need to be accounted for regardless. $\endgroup$
    – fstanis
    Aug 13 at 18:43

25 Answers 25

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A better question would be how to assault it using the portal. It seems like an unwinnable scenario for the attacker in my eyes.

Your "airport" has only one literal gate. To make sure everything goes as planned you likely have several rooms spiraling outwards to control the flow of traffic. Some rooms are for export of goods, some are import of goods with lots of checks, others are in/outflow of foot traffic. But all end up in the central portal chamber where they (somehow) are teleported to their destination or received and immediately led towards one of the outer rooms to make sure traffic can keep flowing.

Any army that teleports there will teleport into a room you control, surrounded by more rooms you control. This is basically the worst nightmare for any attacker: you appear right outside of the walls and can only attack one section while you are surrounded by said walls.

  • the room will likely be large enough for gates to be closed before the attackers can reach them.
  • even if attackers reach the outer rooms, those rooms will also be closed off.
  • the ceiling will be high and filled with murder-holes, allowing rocks, boiling water and all that good stuff to be dropped onto the army below with little risk to the defenders.
  • you could have siege equipment nearby, ready to be loaded and fired into the room at your leisure. Just imagine a few ballistas that fire through a narrow tunnel at midriff height in the walls, limiting the space the attackers can stand on.
  • as part of the control of traffic a series of knee-high walls could be in place, these would incidentally make it a lot harder for attackers to bring and use siege equipment.
  • the portal can be lowered into the ground.
  • you could drop a few ton of flour and sawdust into the room, then light it, preferably from a distance. Dust explosions are incredibly dangerous in enclosed spaces.

There are any number of more things you could imagine. Holes in the floor for archers or spearmen to stab enemies through? A series of arrow slits for your defenders to rain death from? Indiana Jones style rolling rocks? Tilted floors to make any fight an uphill battle? Perhaps the portal itself is in a relatively small room and there are simply many hallways connecting to it, limiting the amount of people the attackers could bring in once those doors close?

In the end its pretty much a death sentence to try and attack such a fortification.

The only coordinated attack possible would be to smuggle soldiers through security one by one for months on end, then try to assault the defenders and take the fortifications so you can bring in the rest of your army. But then again, those same defenses will be hard to breach from the inside as well.

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    $\begingroup$ While this question is full of amazing creative answers and excellent points, I'm accepting yours simply because it's most versatile: it requires no special rules or conditions in how magic and teleportation work and isn't tied to a city's specific environment, so it wouldn't be hard for the reader to accept and assume every major "portal" is constructed in this way after the concept is introduced in the story. $\endgroup$
    – fstanis
    Aug 13 at 2:27
  • $\begingroup$ Subterfuge, infiltration or weapons of mass destruction (bomb, biological attack, fireball, huge mass of lava etc.) seem to be the only weakness. Basically things which can’t be prevented by simply closing the gate fast enough. $\endgroup$
    – Michael
    Aug 13 at 11:58
  • $\begingroup$ @Michael that assumes that the technology levels are there, that the "air"port cannot be herimetically sealed (we can make tanks NBC proof) and most importantly that no ripcord spells are in place in the event of a bomb or similar. For example a "return to sender" spell which sends the entire contents of the portal room back to the original sender (if any) and then instantly closes the portal for further use. Also since the point is "invasion" and not "annihilate everyone inside one room of an airport" I doubt that any viral or bomb attack would be accepted by the global community. $\endgroup$
    – Demigan
    Aug 13 at 13:16
  • $\begingroup$ Make the ceiling the floor of a giant aquarium. So in normal times visitors get a nice view of some fishes. But in an emergency the glass is broken and floods the room. $\endgroup$
    – Rob
    Aug 13 at 16:52
  • $\begingroup$ This solution assumes that a small army is constantly on standby, ready to defend the gate. Most other solutions in this thread would require maybe 10-15 guards on duty at a time, at most. (That said, none of the solutions in this thread are mutually exclusive, so a person can theoretically use all of them.) $\endgroup$ Aug 15 at 8:20
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You are number [571] in the queue

To avoid overcrowding at this central portal, every person who teleports in is placed in a first in first out queue. The portal takes a certain amount of people in the queue in an interval and spits them out. In low usage times you might not be put on the queue at all, but in high usage times you will be delayed a few minutes. If there is an attack, then the portal no longer accepts teleports, and the queue is gone through one at a time until all the attackers are dead, and all the civilians are out of the queue. So long as the maximum people in the portal chamber can be killed by the gaurds, there is no way to overwhelm them.

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    $\begingroup$ Interestingly enough, there are internet chatrooms which use a similar system. The administrator of the chatroom can set a threshold T such that the server will refuse any new connections if there are already T users connected. Then, the administrator can program a bot that will periodically set the threshold to N+3, where N is the current number of connected users. At any given time, no more than 3 users can join until the bot's next "refresh" of the threshold. $\endgroup$
    – Stef
    Aug 12 at 12:50
  • $\begingroup$ While this is really neat and simple for human-centric transport, I'm having trouble imagining how it would work for transporting objects, e.g. would a food crate be automatically placed in a queue behind the last person and require someone to move it? Or would different rules apply for it? These aren't unsolvable problems, but figuring out how a magical queue work has a lot of nuisance to it and can end up being complicated to explain. $\endgroup$
    – fstanis
    Aug 13 at 2:31
  • $\begingroup$ If you are transporting yourself and one thing, then you are put on the queue and not let in until there is room for two teleports. If you can't solve this problem then people will teleport into each other, so teleporting won't work. $\endgroup$ Aug 13 at 5:40
  • $\begingroup$ This would be my solution. Logically, you cant teleport infinite people into one place/room, so there has to be a limit. You solve the "nonhuman boxes" issue by having teleportation only work if its into air or other easily displaced material, which people and equipment arent. So you teleport in, and the next persons teleport wont work till you're clear of the landing zone. Staff are on hand to move any inanimate objects that arrive, for security and onward collection. Guards merely have to control security at the point that people/objects leave the landing zone,and not allow dawdling there $\endgroup$
    – Stilez
    Aug 13 at 6:58
  • $\begingroup$ umm .. so the attackers masquerade as legit travellers and pass thru the queue without problem. The city guard has no way to know the motivation of the people in the queue do they? So the strike force gets in, grabs up the weapons that were already smuggled in, or improvises weapons with what they can make or steal, and away they go. If there is a method of access, any method, then attackers can suborn it to bypass security somehow. $\endgroup$
    – vulcan_
    Aug 13 at 7:41
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City in the Sky

You already solved your own problem. Have the portal in a room with trapdoors. At the first sign of trouble, simply pull the lever and everyone in the room gets a crash course on sky diving.

For a land based city, a deadly trap would have the same effect. Crushing roof, impaling spikes, flaming oil etc.

People can teleport in but they teleport into a killbox so they need to get out before they can overwhelm the guards.

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    $\begingroup$ A "crash" course on sky diving. Nice! :) $\endgroup$ Aug 12 at 12:52
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    $\begingroup$ Executing people on the first sign of trouble seems a bit excessive. It's akin to shooting people whenever a metal detector goes off. But having a sort of high-security quarantine room around the portal seems like a good idea. Though maybe killing people easily fits into OPs society (if it is a bit authoritarian and ruthless). $\endgroup$
    – tim
    Aug 12 at 14:32
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    $\begingroup$ If it's the main point of entry for all traffic, how would you avoid collateral damage? Do the innocent travelers in the waiting room also get thrown out in that scenario? $\endgroup$ Aug 12 at 15:22
  • $\begingroup$ This is a simple solution, but has some flaws. Other than the potential for collateral damage @AlexandreAubrey pointed out, some of these traps (e.g. flaming oil and collapsing roof) are costly to "reset" and maintain. I can imagine a determined attacker sending a "suicide squad" first and then the real invading force shortly after, while the dust is still settling. $\endgroup$
    – fstanis
    Aug 13 at 2:52
  • $\begingroup$ this defence is negated by the attackers sending a bomb through unaccompanied. Then either the bomb was big enough to destroy the portal and the city is cut off, or the bomb just clears the defences and the attackers come through to start fighting to their objective. Bigger issue: this defence only works, for sure, once. After word gets out attackers bring things that allow them to cling to the ceiling, or to the trap doors themselves, and attack anyway. Presuming that the builders of the trap don't blab once it is built. $\endgroup$
    – vulcan_
    Aug 13 at 7:30
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If this city is floating, on a high mountain or on an island, have the portal suspended over a long drop outside the city's defences against airborne attacks, accessible only via a long, narrow bridge. When necessary, a hoist can be swung out to lift goods into the city, but if the portal is used as the point of access for an assault, the attackers will find themselves on a platform, needing to cross the narrow bridge (whose balustrades may be made to be able to collapse by the defenders, making fatal falls highly likely) while potentially under fire from the defenders manning the main fortifications.

In order to prevent the attackers blockading the teleport target, the city should have at least three teleport targets, only one of which would be active at any time. In addition to deactivating a teleport target, these suspended target platforms could be made to be able to be physically turned upside down, so that once so turned, any attackers would be literally dropped from the platform.

There is always the problem of social engineering... attackers could kidnap some important people and try to use them as hostages in order to force entry or maintain a blockade, though even this has a potential solution, though it would require a siege mentality and a minimum of decades to prepare the populace:

Establish tradition and law in the city that hostages are legally dead, and that to be taken hostage or kidnapped is no different from being murdered, even if the body is still moving and talking or otherwise displaying the illusion of life. The hostage taker or kidnapper is legally responsible for their crime and whatever follows from it... so should hostages be held on a teleport target platform, and the defenders must dump them to their physical deaths along with the attackers, the attackers were responsible, and the hostages were already dead the moment that the attackers captured them, and the defenders were in no way culpable.

The other advantage of declaring a hostage to be dead is that should they escape or be released, they would have to prove their identity and loyalty to the city in order to be declared alive and to be trusted once again. This gives a good, socially acceptable reason to treat a potential Manchurian Candidate with suspicion.

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  • $\begingroup$ Turning platforms upside down as an anti-blockade mechanism is genius. The only counter I could come up with would be if the attackers can themselves float or fly, but allowing for this mechanism opens the door for a lot of other types of attacks that don't involve teleporting in the first place. $\endgroup$
    – fstanis
    Aug 13 at 2:57
  • $\begingroup$ the platforms idea is great, against the first attack. After that word will get out about the rotating platform defence and attackers will just figure out counter measures. The Impossible Mission force would overcome this trick in any number of ways $\endgroup$
    – vulcan_
    Aug 13 at 7:28
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No security is perfect.

For every countermeasure you can think of to stop the invaders, there is a counter-countermeasure.

  • Defenders: guards around the portal. Attackers: send in an elite crew of mages that can overpower the guards.
  • Defenders: trapdoor floor, to drop threats en masse into the sky or a pit trap. Attackers: Make sure your mages can all fly, or one mage can lift everyone with telekinesis.
  • Defenders: massive collapsing ceiling. Attackers: bring your mages in with a strong steel roll cage that can protect them against the ceiling.
  • Defenders: boiling oil. Attackers: a spell to control oil, or freeze the oil.
  • Defenders: put the portal at the bottom of a deep, deep mineshaft, near a lake. In case of attack, let the lake flood the mine. Attackers: water breathing spell, or bringing their own air.

The defenders would also need to worry about bombs or poison gas being teleported in. They would also need to worry about sleeper agents that could come in peacefully one by one over a long period of time, then organize once they are in the city in sufficient numbers.

The defenders also need to worry about portal denial. If the attackers can repeatedly cause enough havoc that the portal must be shut off, then the city will be starved of resources.

An ideal system could work like this.

  • A very limited number of high-level trusted people are given magical keys, that let them teleport in from anywhere.
  • Everyone else can only enter the city through checkpoints in other, friendly cities. The checkpoints have guards that pre-screen people before they can go through. The guards in the magic city are constantly scrying these checkpoints and in communication with them, so that if the checkpoint guards get overwhelmed by attackers, the guards in the magic city can shut off teleportation from that checkpoint only.
  • There are traps at the checkpoints, and traps around the magic city portal.
  • To enter the city from a checkpoint, a guard in the magic city must explicitly take action to allow it, for a brief window, based on his communication and scrying of that checkpoint. The default is that no one comes through.

It can still be compromised. Get enough traitors among the guards in the magic city, and anything is possible. It also does nothing against the sleeper agent plan - for that you need an intelligence service.

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    $\begingroup$ This makes sense if the system of magic in this world is overwhelming - i.e. there are strong mages impervious to all kind of physical attacks. Even in this case, attackers would be at a great disadvantage and can prevail only if the city does not put strong mages on the watch. Anyways, +1 for listing otherwise effective security measures. $\endgroup$
    – Alexander
    Aug 12 at 3:48
  • $\begingroup$ @Alexander as long as you can teleport in from anywhere without pre-vetting, the attackers can repeatedly send a bomb through. Do that often enough and no one can use the portal - because it's not safe, they could be blown up at any moment. $\endgroup$
    – causative
    Aug 12 at 4:02
  • $\begingroup$ if such bombing is possible, then our security problem is solved (sort of :) ) $\endgroup$
    – Alexander
    Aug 12 at 4:08
  • $\begingroup$ @Alexander also the most effective trap would probably be explosives, too. Have the portal area be lined with thick steel walls, with a sealed blast door, and lots of remotely triggered bombs all over the floor, walls, and ceiling for the guards to use in case of a problem. There's very little any but the most OP mages could do to survive an enclosed detonation like that. (Well, they could teleport in inside a thick steel chamber of their own, that might resist the blast - but then they can't open the chamber or move without getting killed. And the chamber might not protect them anyway). $\endgroup$
    – causative
    Aug 12 at 4:19
  • $\begingroup$ I'm not sure if "checkpoint in other city" approach works. Other than it changing the original premise of how teleporting works, an attacker could either overwhelm the other city via conventional warfare or ally themselves with it (or both). This wouldn't necessarily give them access to the city behind the portal, but makes a blockade very easy. $\endgroup$
    – fstanis
    Aug 13 at 2:38
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Only Activate the portal when needed

Have the portal deactivated most of the time. Whenever someone needs to enter the city, they have to (magically?) send a message to customs and get approved for a time-slot. They're given a ~10 second window to teleport in before the portal is deactivated again. It's not particularly fast, and if there's a lot of people coming in there's a high chance that attempting to teleport will still get you in during someone else's window, but the guards will be better able to recognize problems (approved for 2 people and a crate but 5 people show up, everyone gets detained).

This doesn't prevent a careful strike team, but that could be limited by making the portal room smaller, so that only a few people can fit in it at a time. If they need to teleport a larger group, maybe one of the walls is movable, or there's a second portal that's always deactivated except when a large group needs to be teleported all at once.

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  • $\begingroup$ I think this complicates city's regular operation a lot, but doesn't necessarily prevent the issue - as you mention, 50 people could teleport instead of 1 in that 10 seconds window. $\endgroup$
    – fstanis
    Aug 12 at 14:43
  • $\begingroup$ @fstanis people have been defending choke points for a LONG, LONG, LONG time. We're quite good at it. You just need to calculate how many attackers can practically come in at the same time, and have enough defenders aiming mounted weapons at the portal so that the attackers are caught by crossfire without the defenders shooting each other. $\endgroup$
    – RonJohn
    Aug 12 at 16:25
  • $\begingroup$ @fstanis as far as complicating "regular operation"... hardly. You schedule incoming transfers from known portals, and have your mages keep an eye on those other portals. $\endgroup$
    – RonJohn
    Aug 12 at 16:27
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Security checkpoints

Create a small station, isolated from the rest of the city (perhaps on a separate floating island or whatever), that anyone can teleport to. There, people are screened and confirmed to be little or no risk before being allowed to teleport on to the main city, which can only be teleported to via an authorized point on the station.

Only having one portal is somewhat of a choke point as well, so with this scheme you can have as many stations as you wish, as well as multiple portals on the city itself. Each station has a designated load it can support, and the portals in the city can only be accessed from the station.

Stations can be secured from hijacking via various measures to verify people before they are allowed to teleport on. Among these could be requiring the guards to enter passwords or scan their fingerprint/face/etc.

One final bonus: Having multiple portal points in the city means that while everyone teleports to one (or multiple) stations to make security easily, they can emerge at various points in the city, so you don't have people having to walk across the whole city to get to their final destinations.

Hope that helps!

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  • $\begingroup$ I like this last idea of having multiple portals in the same city, but not knowing in advance which one you'd get routed to. It may not solve all problems, but it's simple and makes an attack much harder to coordinate. $\endgroup$
    – fstanis
    Aug 13 at 2:01
  • $\begingroup$ This is pretty much the answer I would give. The single biggest point of failure in the Stargate series was always the fact that, even years after starting the program, Earth's portal room (or Atlantis') was a single point of failure. They could have simply established one or more forward bases offworld where people are quarantined, etc. and many of the foothold situations would have been averted. It also prevents WMDs from destroying Earth. In this question's case, a nuke might still take out the city, but any normal bomb would just take out an entry point. $\endgroup$
    – MichaelS
    Aug 14 at 5:12
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A thought inspired by real-world security: The key is a magical incantation that is determined by the portal itself, where the portal will reveal the code to the security on the portal side and security can evaluate whether to share the code with the applicant or not prior to their attempt to portal.

To portal in, the applicant needs to do a request-entry spell which will allow the city's security magic to scan their identity and location as well as some information on their surroundings, like what they have on their person and anything in a specific distance around them. This information is passed by the portal to mages on security, and a passcode for a single-use entry is generated for the applicant. If the mages on security are OK, they can share the code with the applicant, incorporating that code into the spell will allow the teleportation to work. The portal is configured that only the individual that cast the request-entry spell can use that code, anyone else using that code will be ignored. This would allow for pre-screening, and nobody knows in advance what the code will be since it is generated based off of information provided by the request-entry spell; and even if the code is shared to someone else it is useless because it is tied to the identity and time of the request. (inspired by SSL) The overall process is relatively time-consuming, but not necessarily more so than other customs processes, and makes forced entry near impossible.

Security measures like a kill zone etc. should still be in place in case someone is somehow able to use magic to pass false information to the magical scan, unless that magical scan is somehow "un-hackable".

Another note, relying on a single point of entry like this leads to another weakness, which is, if security becomes convinced the portal is the only way to get in could cause defensive measures to be relaxed against non-conventional assaults. A group of insurgents could come in under false pretexts and build their own portal inside the city, so any materials or incantations relating to portal building would need to be strictly controlled (which could be prohibitively difficult if the portal building material is commonly used for other magical purposes); Or, insurgents could just attempt to access "the hard way" by flying, digging etc.

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In the SG-1 series ( Stargate franchise ) the stargate require few minutes for activations, so on each side can prepare on a possible "passage". Also on the earth side, there is a protection and a way to close the stargate so an invasion or a not-scheduled passage can be blocked.

So an attacker cannot pass unnoticed, and large scale invasion are not possible.

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You can solve the problem with the "key" method by changing the type of "key" on a monthly basis, so that stealing a bunch over time doesn't work - keys rapidly become outdated.

As Allan pointed out, you can also limit the number of teleportations per unit time.

Alternatively, you can have the portal only open at certain times.

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  • $\begingroup$ I've thought about different additional limitations to the "key" mechanic (limiting the number, making them expire), but all of these can plausibly be countered if the attackers are e.g. a group of otherwise legitimate visitors who turned evil, but kept their keys. Or if whatever creates these keys is compromised (e.g. bribing a keysmith). $\endgroup$
    – fstanis
    Aug 12 at 14:29
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Lava traps.

Lava is able to solve most problems. Simply set up the portal room so it's in a fortified area with heavy teleport wards and set it so that someone outside the entry room can trigger a trap to dispel magic shields and drop several tons of lava ontop of everyone inside the room.

If they survive just bombard them with artillery magic from afar till they die.

Limit the number of transports per second

Have each portal only allow in so many people, not enough to overwhelm the guards and the traps. If you have 6 people allowed in a minute (enough time for them to be scanned for weapons and bombs) that's enough time to let 10,000 people enter or leave. Once they pass the entrance scan, they can go to the more traditional airport security checks.

If security is heightened for some reason, you can only allow in 6 people per ten minutes. That gives enough time for a more detailed search and scan, and still allows 1000 people to enter.

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It's an interesting scenario, but it's a city. People need to come in and out all the time. Food needs to make its way in, and so does water and so do other basic necessities. Anyone can attack a city from within, it just takes longer. Move a bunch of people in one at a time over a period of a few months (or years) and then, at a prearranged time, have them converge on the portal room / city gates. It's been done since people learned how to build giant horses out of wooden planks.

The only way you can defend a city over the long term is by having it in a country that's fairly secure. An isolated city is pretty much undefendable.

A self-sufficient fortress, on the other hand, might be more defendable. I don't know what magic does in your world, but if it can attend to the necessities of the defenders over the long haul, all you need to do is suspect an attack, shut down the portal and live happily ever after.

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  • $\begingroup$ This is a good point, but my goal isn't for the city to be impervious to any type of attack - I just want to make sure the teleportation mechanism isn't adding any new ways of attack that a non-magical city wouldn't have, as I fear that would distract and reduce suspension of disbelief ("why don't the enemies just teleport"). $\endgroup$
    – fstanis
    Aug 12 at 14:27
  • $\begingroup$ Ah, well. In that case, as far as I can see, the portal is just a fancy city gate and you're in the clear. :) As in, if the portal is in the middle of a large public plaza, that's the same as having a wide fairway entering the city, and if the portal is in a tight room, that's the same as having a "very" secure gate (for different values of "very"...). $\endgroup$ Aug 12 at 15:53
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Use a "firewall" -> Prevent certain unknown or even unwanted individuals from porting into your city. Set up rules to who can or cannot teleport.

You could argue that, when teleportation is possible, the city could have some sort of telepathic link up running, sou you'd have to "call an operator" that whitelists you, like on an airport f.e. [ but since I don't know how you handle telephatic communication in your world, maybe this isn't for you ]

You could only allow teleportation FROM a well-known spot so that you could setup guards and checkpoints there.

Ultimatively, the harder something is to be reached, the more likely it will be it's attacked from the inside. While mages could not teleport there all at once, what exactly stops them from coming 1 per hour or something in the likes?

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The simplest solution would be to create choke points. Enclose the portal in a building which is sufficiently strong (physically and/or magically) to make a destructive or tunneling escape infeasible. Place the gate at one end of a hallway sufficiently long to allow security to recognize a flood attack and drop a portcullis in front of the gate before significant numbers of attackers can get through. (also drop a portcullis at the city end of the hallway to trap the attackers that did get in) Don't make the gate or hallway any wider than it needs to be for carts to get through, so as to limit the number of attackers that can be part of any beachhead established (so as to limit the number of participants in a destructive or tunneling escape). This will also limit the flow rate of attackers if the city loses control of the gate containment zone.

(A variation on the two portcullis set up would be to only ever allow one to be open at a time, so that people exiting the gate need positive action by the guards to actually enter the city and the gate is blocked when travelers are being allowed through the city end portcullis)

For a city of appreciable size, there would probably need to be multiple gates to allow sufficient throughput to satisfy your citizens needs. In order to avoid massive congestion at the gates, you would want to spread them around in your city. Set up some form of (magical or mundane) communication between gates so that if one is attacked, the others can take preemptive measures before they even see the attack arrive.

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  • $\begingroup$ This method of defence was created over two thousand years ago and is a fundamental property of castle design. Castles stopped working once attackers were willing to bear the cost of siege engines to be able to prosecute attacks .. or to risk the lives of their sappers digging under the foundation of the walls. Simple bypass is to just transport a really big bomb thru the portal first. OR if all packages have to be carried in send one guy with a few 5 kg shaped charges. He can portal back out before the bombs go off if you don't like the suicide bomber approach. $\endgroup$
    – vulcan_
    Aug 13 at 7:22
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Scheduling For Success

The portal opens an closes based on a schedule. Access to the portal is controlled via the schedule.

The portal is open from location xyz for a short amount of time then it closes. It continues through a schedule of open and closing and cycles back to location xyz after a day, a week, a year, or something reasonable.

One Bad Apple

The portal opens and closes per the schedule. If, for any reason, there is a problem with a source location, that location is forever removed from the schedule.

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    $\begingroup$ This would change the premise of how teleportation works in this world (magic users can teleport from anywhere, but only to certain locations). $\endgroup$
    – fstanis
    Aug 13 at 2:12
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Your real problem is that the portal changes the topology of warfare. What makes ancient war difficult is that you have to muster an army, get that army to where you want to attack, maintain supply for that army, and overcome resistance as you move around towards your objective.

This portal makes getting your army to the target really easy.

The British empire's dominance of the oceans meant its relatively tiny military force could be dropped off almost anywhere on the coastlines of the world, and be backed by heavy artillery from it ships. It turned this advantage into the largest empire the world has ever known.

Your island city basically has an undefendable port -- there is no way to notice the transports arriving -- that any force with enough mages can arrive with an army at. The larger the bandwidth of the portal, the larger the army that can arrive.

If the portal is only rarely used, then it can be secure.

If the portal is regularly used for high traffic trade, then it cannot be secure.

Surround the portal with defences, pointing towards the portal. Inside that defensive ring you have your clearing of customs and unloading of trade goods and the like.

That area is secure, in that it has police officers in it. Near the portal itself, you'd have another layer of defence; the ability to shut off the portal, and some local defences to buy time in the event of a major attack.

What you need is layers. A first layer that makes a small attack stupid -- the guards at the portal itself. A second layer that makes a medium sized attack stupid -- the forces within the portal customs area, and the people who can shut down the portal. A final layer that makes a massive assault stupid -- the equivalent of naval cannons aimed at the customs area, permitting a scorched-earth response in the event of a major invasion.

The fun part is that each of these layers will be subject to capabilities drift and decay. Only the layers that are tested regularly by hostile parties will work as well as expected, and those would be easy to study.

Also, the political costs of having the equivalent of huge naval cannons aimed at the merchants inside of the redoubt will grow.


Ideally, the ability to make keyed portals that only work from other portals would exist. Then you'd build land-based gateways, from which you can send things up to the island.

These land-based gateways could control the approaches around it, and prevent the ability to mass an army and teleport right next to your city in the sky. If and when a land-based gateway is under threat, you'd start massing countermeasures, or even shut down the gateway entirely.

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This is a very interesting question! I propose a few combined measures in an attempt to make a bulletproof system:

  1. Isolate your teleported from the city proper, ideally by a small chokepoint which can be severed in case of emergency. For example, picture the Rainbow Bridge from the Thor movies.
  2. Everyone teleporting must enter a stack and wait their turn in limbo before they are allowed to enter.
  3. Magical pre-scanning. When people enter the teleporter, wizards scan them and assess their threat level. These wizards teleport weapons, magic artifacts, and other potential threats into a "Baggage Room". You can even teleport magically powerful travelers or warriors into a quarantine room for further screening.
  4. Some sort of ID system. Essentially, citizens (who present a valid magical ID), enter the short queue. Outsiders who are not pre-cleared are sorted into random subrooms, and wait for longer.
  5. A limbo room for added security. This should be a completed disconnected room (maybe a pocket dimension) where people are sorted and prove they are not threats. At the push of a button, any official in this room can sever the portal into the city itself.
  6. Some sort of antimagic field / spell within the subroom and main room. That will keep people from playing hijinx.
  7. Keep a wizard busy noting where people are teleporting in from. If a large group suddenly teleports in from a rival country, they can block further entrance from that area.

Another option is to only link with valid, trusted portals. They could use a sort of magical public key / private key system to authenticate. However, this isn't as flexible. I hope this helps!

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I wanted to share the solution I'll likely go with. There's some fantastic answers here and this idea draws heavily from points others here made (thanks, y'all!). It's by no means perfect, but fits well into other aspects of my world and I wanted to share it in case anyone else can benefit from the idea.

Teleportation can be undone until you leave the portal area

A portal, when open, creates a bubble that's a few meters in diameter (presumably, exact size can be tweaked during construction). Teleporting into it puts you in the middle of this bubble.

When the portal is closed, anything that arrived and hasn't yet left the bubble is "booted out" - the teleportation is undone and the person or object is teleported back to where they were prior to their attempt. This gives guards an easy, non-lethal way of dealing with a problematic situation that starts developing.

A few other thoughts on how this could work:

  • The portal can be immediately reopened and continue normal operation.
  • Objects larger than 1/2 of the bubble's volume can't teleport into it.
  • I'm not sure what would happen if someone partially leaves the bubble, but I'd like to avoid a mechanism that would make people lose their limbs this way. One idea is to make the bubble have a surface tension and behave somewhat like a soap bubble, where it slightly extends when an object is leaving.
  • Anyone can just walk into the bubble and wouldn't be affected if it's closed, this effect applies only to things that arrived by teleporting. This allows guards to inspect objects and interrogate people, if needed.

In practice, cities would probably want to build infrastructure that makes good use of this mechanism e.g.:

  • The bubble can be fully contained in a room that's bomb-proof, magic-proof etc.
  • The area where the bubble is has limited exits that can be used as choke points.
  • The bubble can be in a location that's hard to leave on your own, e.g. suspended in the air on a platform, at the bottom of a large hole or something similar.

There are two alternatives / additions to this that I'm also considering:

  • The bubble can somehow block or greatly reduce the effect of magic inside of it. This could prevent possible magical attacks and puts arrivals in a vulnerable position. As teleportation is also magical, this has the interesting side-effect of "trapping" arrivals until they're either cleared to enter the city or kicked out.

  • Arrivals in the bubble can be incorporeal until they leave the bubble. This solves the problem of sending a bomb through and reduces risk for those arriving. It also removes the issue of the portal being "full". However, it also raises new questions around how being incorporeal works and how exiting rematerializes you.

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well, you have lots of input on this already, but some pragmatic points

there will never be just one portal.

reason 1: the need to bring in freight requires quite a different design than for people. Food needs to come in a continuous stream that cannot be interrupted for irregular personnel transport, and big things need a portal much larger than a person that will thus be much more expensive to operate.

reason 2: differences in rank of people arriving mean corresponding levels of finery in the arrival chambers.

reason 3. you will never avoid the power of money. The rich and powerful will want private entrances that give them the privacy they need to take advantage of their inferiors.

reason 4. security or intelligence organisations will need portals they control for clandestine access.

My conclusion is the most effective attack will be a fifth column or Trojan horse variation. Smuggle the people who can build a new portal into the city, or ship in something big enough to contain a pre-fabricated portal, set it up in a space big enough for a commando team assemble and prepare for their attack(s).

Using a portal that handles regular traffic will have many bigger problems to overcome for an attack using it (them) than the difficulty of getting a pirate portal into operation.

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  • $\begingroup$ I think making building portals extremely costly makes a fifth column attack infeasible: if you have the resources to do so, pillaging a city is unlikely to be a priority to you while destruction and havoc are easier via a series of hefty bribes. $\endgroup$
    – fstanis
    Aug 13 at 2:46
  • $\begingroup$ @fstanis the cost of any form of attack will be costly. In the context of this question the need for an attack has been decided. The answer has to be about choosing the "best" way to prosecute the attack. If pillaging is the purpose there are of course simpler methods for economic warfare with no need to invade. If killing off the population is the goal then block all the other portals that handle traffic into the city and starve them out. the 5th col attack just bypasses all issues of how to get a viable strike force into the city. With need established cost is irrelevant $\endgroup$
    – vulcan_
    Aug 13 at 7:10
  • $\begingroup$ While not exactly the same, I'd compare this scenario to building a beachhead right in the middle of a large coastal city to mount an invasion. It's not impossible nor a bad idea per se, but the point at which it becomes feasible to do that is the point where the city is already largely subdued. $\endgroup$
    – fstanis
    Aug 13 at 19:20
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It seems to me that, by default, you can not have two entities teleport in simultaneously — that, if they did, something terrible would happen, such as them occupying the same space and both/each ending up as entirely something else. That is, it must not be possible. (If, conversely, the portal did support simultaneous arrivals, it would be very dangerous.)

Thus, if this is a stable portal used for a high frequency of transfers (or maybe even a low one), it is going to have to be set up to support only one transfer at a time. (Since mages can teleport from anywhere, it is not apparent how any management system could be used… apart from an exclusive “now” token magically offered by the portal. The outcome is the same; simultaneous arrival is bad.)

(I suppose that you could have it that the internal workings of the portal mean that things arriving simultaneously (from different places) exclude each other physically in the same way as in normal space… but then you are discarding this useful constraint.)

The rest is obvious; mages simply can not arrive simultaneously (and if they contrive to anyway, it will not go well for them).

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All an attacker has to do is send in one person infected with a highly contagious lethal disease, and this city could be toast. They wouldn't even have to send a person - just an unassuming small package covered with the disease.

Attackers could also perform a denial of service attack. So many people try to crowd the portal at once that no one can get through. If this is the best (only?) way into or out of the city, it could pose a serious problem.

And as you mentioned, siege. If there's only one way in and the outsiders can figure out how to block it, everyone inside will starve in short order.

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  • $\begingroup$ Bioweapons... generally banned. Anyways, where would you get a highly contagious lethal disease on demand? They're not sitting around in a scientist's lab waiting to be used. $\endgroup$ Aug 17 at 8:12
  • $\begingroup$ Generally banned by whom? This is a fantasy world. If they can make teleporters into mountain top cities they can probably figure out bioweapons. $\endgroup$
    – Truth
    Sep 5 at 21:39
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Security of any sort is essentially an arms race. The defender is trying to make it more costly for the attacker to succeed. The higher the value of the target, the more motivated the attacker, and thus the more costly the defender has to make the attack. This applies equally to medieval siege warfare as to contemporary network security.

Pretty much all the other answers here illustrate this principle.

However, the "value of the target" is in the eye of the attacker. So if the attacker's goal was (for example) assassination, defenses against an all-out attack, or even a raid, would be ineffective.

One way to increase the cost to almost any attacker would be to make closing the portal also shut down the ability for mages in the region to teleport away to other portals. This would mean that any attack on the city would almost certainly be suicidal, since the attacking force could then neither retreat, nor expect reinforcements.

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You want a small defending force to be able to suppress a much larger attacking force. And, you have total control of the surrounding architecture.

Or, "Tucker's Kobolds" design an airport!

Defensive fortifications: Create a gauntlet (maze)

To exit the portal area, everyone must take a serpentine path for 2km (1 mile) to exit the teleport area. For most people, it's just the "airport shuffle" we all know. Walk a mile and you're out.

But for invaders, it's a gauntlet. The entire route is fortified, impenetrable, and lined with weapons placements appropriate to the age/tech. Boiling oil, machine gun nests, whatever.

But worse, everywhere in the gauntlet is in enfilade to the weapons emplacements all the time - fully exposed with no hiding places - no cover. While the defenders sit in their fortifications, laying waste to the attackers with impunity.

Of course the gauntlet is decorated, and festooned with food carts and vendors, to make visitors feel welcome and so it isn't obviously a killing zone. But all that stuff, from the information boards to the mead vendor, is designed to be utterly flimsy, or even booby-trapped. So that it's useless - or worse than useless - as cover.

So at the first hint of trouble, the port sounds the alarm and everybody from warriors to food vendors steps up to the battlements and rains hellfire onto the invaders.

Regular travelers get features to break up the seemingly long walk. For instance after 1/4 the distance, there's Immigration. Another 1/4 there's Customs. A quarter after that there's money changers, tourist info, anything to make people stop and catch their breath.

Stop sprinters, though

The only attack plan that makes sense is to sprint the entire run, do nothing to defend (no place to hide anyway). And you don't want it to be a "foot race" between attackers sprinting the gauntlet vs. defenders coming to readiness.

So you need to slow attackers down. Make them weave around rolled-down logs, spike traps, fires, anything you can do. Those things aren't there to kill them, just slow them down to increase they are in enfilade, and give the defenders longer to crew up if that's an issue.

The design of the vendors could well be part of that - think "like an IKEA store".

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The portal is disposable, after it is used, the entire portal is itself teleported to a destruction portal built upside down over a volcano, from which the disposable portal tumbles down into the flames.

Therefore all key are one time keys.

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The main weakness of a city that's difficult to enter is, it's conversely easy to besiege. Even if no attacker could get in, cutting off the only supply route into the city would be very easy, with potentially devastating consequences.

Any kind of unique entry point can easily be disabled. Attackers could teleport all sorts of nasty stuff into your portal whenever they pleased (giant spiders, poisonous goo, bombs...) and render it so bloody dangerous a potential visitor would think twice before using it and the flow of goods would be hopelessly disrupted.

Besides, since teleportation is possible from anywhere but the destination requires a portal, the attackers would be practically impossible to stop. Even if you added some rule to your magic system allowing to trace back the origin of the teleport, the attackers would only have to move to some remote location, giving them ample time to scamper off before you could come at them yelling and panting from the nearest available portal :D.
So a determined attacker could disable your portal for days or months on end without any practical means for you to retaliate or put a stop to it.

To avoid such a disaster, you would need serious stockpiles and other, severely controlled and restricted portals dedicated to vital supply, only manned by trusted guards and logistics workers, to make sure your citizen would still have enough to drink and eat regardless of the availability of the "visitors" portal.
Plus maybe some emergency exits for a last resort evacuation of the whole population, since teleportation is only available to skilled mages.

Now to answer your question, I would create intermediate checkpoints.
A checkpoint would be a simple room in some remote location (maybe deep underground or in the middle of the sky) containing three portals:

  • the arrival portal, that would be switched off while visitors and/or goods are inspected
  • an exit portal sending the contents of the room into the city.
  • another exit portal sending the contents of the room some safe faraway place

These exit portals would be under the control of some guards and inaccessible directly (well, that's what magic is for, isn't it?) so nobody could set foot in your city without the assent of a guard.

If your magic allows, the guards could operate these rooms remotely (using astral projections, summoned creatures, crystal balls...), controlling the flow in and out in total safety, blocking the arrivals long enough to perform any check they like, then allowing harmless visitors and goods into the city and sending anything suspicious to some meadow in the middle of nowhere or a bottomless pit, depending on how much you value the life of a hapless civilian caught in the crossfire.

At best the attackers could blow up a checkpoint by sending some kind of bomb set to explode immediately after teleportation, but that would still not harm the city itself.

In a world with cheaper magic, you'd need to have guards physically present, willing to be blown up with the checkpoint, which might reduce the efficiency of the system (a cowardly guard might be threatened into letting an attacker in).

You could multiply these checkpoints to adapt the throughput of your system, possibly having a row of portals leading into your city, like a supermarket checkout.

Obviously any kind of undetectable attack (poison, disease, traitors, well hidden bomb or weapons, sleeper agents...) would still have some chance of success, no matter how many checkpoints you pile on top of each other.

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