How can one plan structures and fortifications in advance to help regaining control over their city walls?

Question

In my fantasy world, I have a really, really big city where trade and people flows are done through magic gateways. Partly as to not let whole districts suffocate in poverty (among other bad things), but also because it is basically the only way to reach it. The gateways are made -and maintained continuously- by trained mages and artifacts, and link two realms of reality together. A mage alone can open a portal enough to make a rider and its horse pass for a short time, 3 or 4 can make whole carts get through them. Artifacts on their side, mostly help in maintaining the portal for longer durations (think hours). Most models are not that big, though they cannot be carried in a pocket.

Now! Now! A big, rich trade center is the source of all... "interested" and, uh... "unwanted" neighbors (well, sort of, they're not physically next to the city), so a wise ruler need to plan some defensive structures in case things go south. Especially if an army manages to make a breach somehow and take over portal stations. Or create their new ones by sabotaging whatever "I don't like portals" gizmo in the city.

In order to prevent this (and because of its history), the town is divided in several quarters, separated by strong, fatty walls¹. These structures are capable of repelling intruders from one side then another², since you don't know from which district they will come from. In other words, the two sides are heavily protected. This means that you can reasonably contain an opponent inside a single district, which is considerably better than letting them spread wherever they want.

All is good and nice until you try to take back the districts you have lost. Indeed, now, you need to attack your walls, protected by your enemies. Ain't that neat.

Therefore, how can one prepare for this eventuality and protect their walls from such robbery, or at least minimize their defensive value?

In particular, I'd wish :

• To reduce damage to buildings, especially the walls, since they are quite expensive in sheer resource quantity. Other potential losses are more acceptable, like sacrificing a squad for the sake of the city.
• To avoid basing the strategy solely on hidden intel (e.g. : secret passage ways). I've thought about this already, and find it quite a risky move if your enemy has spies and hear about them.
• To be an enough general approach to take them back. You mainly have to know there's a threat sitting on your walls and another one behind it.
• The longer they are prevented from using effectively the walls against you, the better it is. Or make them unavailable when you want to take them back.
• Limit to medieval fantasy technology. No electricity, oil or steam, but you can use clever mechanical and chemical concepts (cogs, pulleys, greek fire, even some gases...). While there's magic, use it only for non-lasting enchantments to create your devices or to trigger them. Otherwise that makes too many possible answers, 'specially without any detail on how my magic works.

However, you can plan any kind of structure or strategies you can think of, inside or outside the walls. Also, the town guards can set up something when they notice things are not going well for them, so you have some time to trigger traps or other devices, during a whole day or maybe even two. Indeed, the city is big, and the town's defense is tough and organized. In some way, it is a little like laying siege on fortifications, but you choose your walls and spend one day in them before. Remember that they need to be defendable before the counter-attack!

For instance, one plan I thought of is to build trebuchets on the two sides, then to sabotage the ones in the district you are losing. Afterwards, take back the walls by using the remaining trebuchets on the other side. It's clearly not ideal because of the damage it would do and the efficiency -they're still using the walls against you-, but you have the gist of what you can do and what I am looking for!

So given this configuration and constraints, what better plan than mine would you think of to take back what's rightfully yours, wise ruler :)? Hopefully you can find an answer, this question may not be easy!

Question clarification addendum

I need to point out that what the main difference with traditional besieging lies in is that you can prep things up before the invasion. I know how to lay a medieval siege (well, the common tactics at least), and some basic research would be enough in any case ^^.

In traditional warfare, being able to set things up in advance gives a huge advantage, be it by putting traps or denying key structures or resources (see scorched earth tactics and mine fields for instance). So, what is more interesting is to know how can one help themselves as much as possible if they have some time to prepare things on the fortifications or around for a counter-attack. Put differently, if you have time to prepare things in, on, under and next to the walls, what would you prepare to help you out in regaining these?

Note that even though the easiest idea would be to make purely impregnable walls so that it never happens in the first place, as we all know, you can always create an opening with enough efforts, and you're never too careful :).

Magic Addendum

Since you like very much magic portals, here's some additional intel on how it works : To be perfectly safe, this magic must be cast on a path the caster knows (start+end points), so this limits how many points you can reach with one gate master. Everyone on the gate's path will sense it and any mage can create an interference zone, making it dangerous and unstable by default. Walk blind-sighted along a cliff's border and stop exactly before the fall, on an unstable terrain and while screaming where you are, and that's what traversing such a gate will feel. With a gate expert, you might teleport someone into an interference zone, but it is risky. Also, note that gate mages are not that many, since it is difficult by default and require practice and experience.

Last important bit of information, you may be able to place portals anywhere you want, but some places are much easier to work with than others. Only true gate masters are able to memorize and create portals anywhere, and such are rare and priced

Notes

¹: There are also smaller walls/defenses around the portal stations, if you ask, but they may get overrun by a determined, full force army.

²: One side, then another one. While they "might" withstand a two-way attack, and even though it may be useful to have a safe path between two free districts separated by enemy territories, it's really, really hard to take in attacks from both the front and your back on such a thin line.

Answer 1

Make the Walls like a hive

You make your city in a way that all the district walls are connected to one another, similar to a hive. That way, even if you get intruders in some disctrict, you will always have access to that district's walls.

You could also have only one access to the top of the walls in the entire city, and that would be inside the headquarters of your city guard. The intruders will never have control of your walls, and you'll always do.

Answer 2

I'm not sure I fully understand the issue. What I got is: I built a very strong districts but since it can be taken I'm worried that those defenses can be turned against me in when I want to retake the city. Correct me if I'm wrong.

Same way they got it

I don't think this is a problem at all.

If you are in a position to take the city back then you already got your own dudes in superior numbers around the city ready to take it back.

Starve them out. is he most popular option. You don't need a single trebuchet shot or ladders or rams. Simply surround the place until they are too weak to rebel you.

In fact in history people surrendering if given favorable surrender is very common. Because you don't want to just die. Even hardened soldiers can see the value of being alive if they are given the chance.

If you are worried about the stuff just tell them this: I'll send a single messenger in the city, he will examine the buildings and see what you did. If you did not destroy it or make a mess then I'll grant you your lives once you give the city.

Again this is assuming your own army that outnumbers them 10000/1 is already in control of the country side and you have decimated their own invading force. You can chill out while they starve to death. Sad but true.

Make it rain...dirt

I'll assume you have exhausted all magical means of taking the city. The walls are warded and you can't teleport people there in anyway or use magic to control a guard..etc. Fair enough.

Now what about the "air space" of the city?

Think about it. Wards are probably on the walls, they might even extend a little bit upwards. But you got kilometers of space above the walls.

Gather a bunch of wizards, open a large enough portal on top of the city then shuffle dirt right into the city. Ride the ligh...dirt.

Say a 100 square meters hole would be more than enough.

What's great about dirt is that it is dirt cheap. Please let me have this pun.

You can just get your wizards to the nearest patch of soil and bang you got a near infinite source of fuel.

I'm well aware that dirt has a weight and buildings don't take kindly to tons of added weight. However you don't have to fully submerge the city in an Atlantis of dirty kinda a way. Simply keep adding dirty until they give up. Or...

Shit rain, literally

Have your soldiers gather all that precious filth and even dung and of the entire army and bang. The defenders will surrender after the very first portal opens.

Changing collapsible walls

Instead of having one single spot on a wall that the enemy can hit with a hammer and it falls apart. You will constantly change that position on the wall with each new renovation.

The only person who knows the exact spot is the castellan and the engineer. You can kill the engineer or just move them around or have them be trustworthy. Up to you.

Point is that once the enemy take a district then they have to worry about this sort of weaker wall that they have no reasonable way to detect or know.

Why?

Well. The walls are large enough and only by clever mechanics can the wall be taken down. There is little time to test every single wall of a district let alone the entire city. And the best part is that if they know the spot they have no chance to do anything about it. Like are they expected to figure out a way to rearrange the wall on the fly? build a new wall? Enforce the section?

This sort of things is like what the lawyers talk about as a good question. Whatever the witness on the stand answers it fits the lawyer just fine.

Please notice that the idea here that it is only be very special and clever engineering can the wall be collapsed. Not by having a random bird hit a brick the wrong way. And, again, because of the sheer size of the walls and the chance that it could be anywhere without any signs makes this a strong deterrent.

The hole attackers

OK. We will get back to the name latter but hear me out.

Let's just say that there is this very secret and clever underground bunkers in every district. The bunkers are accessed by different grates and manholes and secret passages all over the city. This means that you have like a hundred safe holes for the valiant knights of backdoor attacking.

Now notice how I'm talking about the size as an important point?

Because even if the enemy knows that those guys exist they have no way of actually handling them. A huge network of tunnels under the city can't be easily flushed out unless the attackers attack it with thousands of soldiers. So. You are not worried about the enemy knowing it. And they work in cells so even if a bunch of holers are compromised you still have others.

The defenders are small in nature, a score would suffice, who work in cells. They memorize the networks like they memorize their names.

Once the city is attacked each cell tasked with a section of the network goes into hiding. They go hide and bid their time for a chance to open the gates for the attackers. Even a double watch at the city gate is not that much of a problem. Soldiers are notorious for sleeping on duty and with say 5 men team you can overpower a couple of guards. And it's the end of the line.

You can also create a distraction. sabotage the enemy's stuff. Spy on them...etc Do something like this and you can just fade to black.

I just hope I understood the question correctly. Tell me if I did not

Answer 3

separated by strong, fatty walls

I feel this is the key: build your barracks into the walls themselves. Let the city's defenders operate directly out of the walls, so that they are always garrisoned and can be reinforced at the earliest instance. Effectively, you force the attackers to lay siege to a city that surrounds them, instead of the other way around. A design like this would not be very efficient for mobilising a large field army, but that doesn't seem particularly important here.

This works because the walls do not need to be climbable from either direction. If you look at the Great Wall for example, you can see that it is bidirectional:

This is the kind of walls you want - not so much "walls" and more like "wall-shaped citadels".

Presumably, there will be passes in the wall connecting the districts. Rather than having a single gate at these location, have two, one facing each district. You can then place the main entrances to the interior and top of the walls in between the gates, since these passes will naturally be the most heavily fortified strongholds in the city.

The walls can never be use against you if you never lose the walls. Not unless you're so hopelessly outmatched that you can't even hold the walls, in which case, all is lost anyway.

Answer 4

It sounds like the enemy would primarily be of a raider-type (smash, grab, and run) rather than attempting to conquer. Any army trying to conquer in such a way, would find itself cut off from resupply and reinforcements and would quickly be overrun.

So basically, do not create unassailable strongpoints within your city, as an enemy gaining control of such a strongpoint (whether through luck or treachery) is creating a massive headache for yourself.

Instead, your walls are simply there to prevent raiders from running in any direction they desire.

Raiders show up, and watchmen in watchtowers raise flags to report an incursion. Local constables/soldiers are ordered to the location and mages set up interdiction so that additional raiders can't arrive and the existing raiders can't escape. The raiders are herded along until they are trapped against a wall and then captured/exterminated.

Answer 5

Research the true fortress design of a "ravelin"... if an attacking force overtakes a ravelin, it can not be used against the defending force because of the exposed rear.

This approach could be used in many different ways.

Answer 6

Many European castles were already designed to address this type of concern. In many castles a section of wall would be terminated at a tower which would then connect the next section of wall, in this way towers would act as choke points where enemies that have captured a wall would be funneled against an easily defended narrow doorway into the tower.

In this way a captured wall still has restricted access so long as the tower entrances are not overwhelmed. Meanwhile the tower itself still has a defensive advantage for archers the fire down upon the breached wall.

Answer 7

Everyday at dusk and at dawn specially trained mages must do the rounds and reset the traps (gas, greek fire, demons, insert your choice here).

These traps could either be actual containers or they could be mini portals.

The traps are hidden in random locations, very cleverly disguised as normal objects and there are never the same amount of traps in each section - the enemy/ everyone would just know that there are potentially too many traps to be found and contained and therefore would be put off even attempting to take a wall section.

If a wall section does get captured by a hostile force, and the mages cannot get there to reset the traps at dawn or dusk, then the gas/ fire/ demons/ army of red headed step children is released and the wall quickly becomes unoccupied again.

Answer 8

The answer is simple:

Portals ahoy!

You have portal tech, right? Well then use it. Here's how: When they take your wall, simply open portals over the wall and first, hit them with knockout gas canisters, or even something more deadly like molotovs, thin their numbers, then drop in troops to retake the wall.

Aireal assault

Another option is to use flight. Have gliders that glide in and swoop in from the air in the dead of night, silent and camoed for nighttime, and then they drop down and retake the wall.

Living wall

Now, this is an absolutely crazy idea, but I think you will like it. You make the wall "alive" using a magical trigger that will cause the top of the wall to bounce and buckle, the unsteadiness will throw the intruders off of the wall.

Hot Sand! Hot Sand!

Make a magical trigger that causes the top of the wall to alight. Burning anyone on top of it into a crisp. There would be hidden flamethrowers in the battlements that, when triggered, will activate covering the whole entire wall with magical flame! Goodbye would-be rebels!

Answer 9

i am not sure i get the detail correctly, though i also agree with @seallussus if you can retake it that mean you have superior force or technologically.

ignoring the cost and materials, my suggestion is to build multiple layers of walls with different height put the outward or the middle layer to be the most tallest to support each wall that was under attack, this including the watchtower height putting roof is optional it can work against you, but it can also be a good lure to make enemy concentrate there, and collapse it when you throw bigger rock there from another layer/section of the wall using your trebuchet or catapult (only add such siege engine on the highest wall layer but if they manage to capture it you practically cant retake it other than destroy the wall) so if enemy taken one section/layer of wall you still have high ground advantage, even better if you can add water on the top, like some kind of roman aqueduct, so you can flood that section if its unsalvageable.

also use simple plank or improvise bridge or if want more fancy and harder to take make 2 opposite drawbridge to connect the other walls for your troop to move on, so if enemy attack and your guards and ppl need to evacuate they can easily getting rid of the plank quickly to disconnect it, or the enemy will need to take extra attempt to take both drawbridge, even better if you add multiple gate in each of the wall path/road to hold or slow them down, but i suggest use bar gate instead so you still can bombard them with projectiles rather than give them a checkpoint to take cover, also make the wall gate or entrance for your guard a bit higher maybe one floor higher with wooden stair or improvise stair to pass through so when enemy attack you can destroy it quickly to make it require extra effort to be entered and give another highground advantage for your guard, and add moat inside and outside the walls perimeter including the gap of each walls, you can use the drawbridge entrance to let the caravan pass through and connect to another section of the walls.

also i assume the portal is inside the town and on the middle section of the quarters/districts right? so they practically get surrounded by the concentric wall, that itself should give the enemy a hard time to defend themselves and protect their siege engine while move closer to the wall, i suggest to clear any building from the portal perimeter for easy/clearer bombardment.

here example image for different multi layer wall height (dont take that to literal)

from:https://www.securityskeptic.com/2011/01/choke-and-screen-firewall-the-modern-day-concentric-castle.html

also here another example from:https://en.wikipedia.org/wiki/Walls_of_Constantinople

my concern is enemy will try to dig pass through or destroy the wall using the tunnel when your guard not notice quickly to prevent it, so i guess you need to pave the town with concrete or plenty of deep stone foundation to make it harder to dig through or fill some section/layers of the moat with water, connect it to river is optional (this include the improvise roman aqueduct defense), i personally against it since that may become weakpoint in this wall defense but that depend on the town design and the wall fortification.

since portal is include here, i doubt you can starve them since they can just teleport their supply using the portal, but only taken one section probably wont be profitable or helpful economically for the enemy outside of raiding or pillaging which is a likely problem than invasion/occupation, since you can just barricade or block trade with them to destroy the economy flow, and they need to deal the same problem of inside invasion/occupation and pillaging too.

since you mention air troop exist, i assume enemy can also have it, so for now, i suggest to create roof bar that cover the entire district to prevent enemy from flying through the portal smoothly, while your troop still can fly and bombard them from above, this also still provide some sunlight and rain, so you still can flood them if overwhelmed.

Answer 10

Typically fortress walls were designed so they had lots of cover from attackers from the outside, but none from fire coming from the inside. All the stairs and ladders (not in a tower) were built open to the interior and the parapets had nothing to hide behind from projectile weapons. So if an enemy gained a section of wall, it offered them no protection from defenders on the inside firing from the strongest part of the fortification, the keep, or from towers.

So if your city was divided into districts, you enclose each in walls that have gaps between them: think of castles built adjacent to each other that have streets in between the outer walls, thus an attacker moving from one from another would need to cross an open killing field and assault another wall. The district walls themselves are built traditionally (open on the inside), with towers or mini-keeps at regular intervals from which defenders can bring direct fire down onto the enemy exposed on the top of the wall.

An attacker moving in the streets between district walls would be exposed to fire from both sides. Taking one district doesn't help you take the next one, because you'd have to assault it all over again. Trying to siege one district would be difficult because you would, by definition, have your back to another one. Defenders, meanwhile, could be shuttled between districts fairly easily: if the coast is clear, you can open the doors on two adjacent districts and allow them to run across the street, then seal up again. Similarly, if an enemy did gain a foothold and looked like they'd be able to take a district, defenders could be evacuated to the next one in a fighting retreat.

Answer 11

You could focus on mental intrigue and/or a way to get people to not care about the walls or their duty to protect them.

Maybe they don't feel motivated to go anywhere else because they've bought into some lies, or maybe they feel like they're so powerful that they think they can't be defeated and don't send the needed armies to maintain the walls- this has happened in real life through history, e.g. getting the guards drunk.

So if you can just keep the armies away from the walls for some time, your army can easily overtake the wall with whatever is available: ladders, overpowering the limited remaining guards, etc.

If you want to make it so you have to have magic to take back the other places, you could make it so that you only share certain magic with certain people, which allows you to perhaps kill or just render unconscious or otherwise subdue the other army with some sort of spell like a knockout gas spell or something. Of course, that could work without magic too with the right technologies.