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The setting is medieval fantasy (for example, D&D) with some magic (although I would prefer solutions without magic).

"The majestic canyon city of Hurr, the gem of the Durr desert or whatever"

City carved into canyon walls (Example image. Source: Pinterest)

The city is entirely built into the "walls" of a canyon (pretty much like the Grand Canyon) which has a flowing river on the bottom.

Most of the buildings are on the side of the "cracks", but some can be constructed on the flat top. Suppose there are a few entrances at ground level (e.g. where the river gets larger and enters the plains outside).

Few to no buildings are built on the bottom, most of those being infrastructure related to water (or walls and gates, in the outermost parts of the city).

Food is gathered via hanging gardens and terraced farms inside the city.

The city came to be because of the humongous quantity of valuable gems and materials present underground in the area. Obviously, this makes it a target for attacks.

So how could such a city effectively defend itself from sieges, with medieval-fantasy technology? (some magic is acceptable, but I don't want stuff like "a wizard makes the city invisible to the outside")

Technology level is comparable to that of D&D dwarfs, so something a little steampunk-ish is acceptable (e.g. they probably have huge dams to control the flow of water, and a good hydraulic system)

What are the biggest threats? (I'm assuming water shortage and an "attack from above", although I'd say the latter would be pretty impractical)

More information:

The "canyon" would look like Mount Conner (Australia) from the outside: Mount Conner, Australia (source)

that is why I say that getting a decently-sized army on top would be impractical at best.

This could be the city's layout, approximately:

Drawing of the city made with Paint (I know it looks awful, but I had to use Paint)

The layout of the canyon itself in the image is random, it's just to give an idea.

Dark gray area is "mountaintop" area, blue is the river, green is the plains outside.

The mountain is connected to a taller mountain range, from which the river originates.

I can see the city having many alternative escape routes for the water, so as to counter any outside attempt at flooding the city.

The two lines in brown are the gates.

Food is harvested from the terraced farms (both on top of the mountain and inside the canyon) and kept in underground granaries.

I could see there being underground escape tunnels (that perhaps allow to bring food to the city undetected)

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    $\begingroup$ I would say its biggest threat would definitely be starving. In that sort of terrain there is no way that you can get ALL the food for the whole city, so you would be importing (export is the gems). Because the buildings are mostly below ground level the enemy just has to push any military from their paltry surface defenses and then sit at the lip of the canyon until they surrender so they dont starve. being inside the canyon it would be very hard to shoot out. Very easy to shoot in. $\endgroup$ – BornToDoStuff Jan 22 '18 at 17:00
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    $\begingroup$ The image you are linking seems to be concept art by Sega, the oldest host I can find without doing any more serious research is this french videogame site: console-toi.fr/archives/28864 ; you might want to consider relinking to that instead of Pinterest :) $\endgroup$ – dot_Sp0T Jan 22 '18 at 17:07
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    $\begingroup$ that is why I say that getting a decently-sized army on top would be impractical at best. The Roman's besieged Masada and this shows that if your enemies really, really want to, they really, really will. During WW1 large areas of the mountains of Europe were a vertical battlefield - difficult is not a deterrence to serious enemies. $\endgroup$ – StephenG Jan 22 '18 at 20:25
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    $\begingroup$ this is a much more defensible canyon city then what you have laid out. i.pinimg.com/originals/15/dc/fe/… consider using it for inspiration. $\endgroup$ – John Jan 23 '18 at 6:26
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    $\begingroup$ Your geologically impossible mesa with a canyon system through it looks like a castle with insanely, impregnably, thick walls that no-one even had to bother building. To attack you need to climb on top (very hard) or go in an entrance (pinch point). Defence is a cinch - until you get hungry. If you felt like using a more traditional (ie: remotely plausible) canyon then stick it in a very big, bad desert - and make the canyon floor the only feasibly navigable ground you can use to get to it. $\endgroup$ – Grimm The Opiner Jan 23 '18 at 12:17

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Defending this canyon city against Siege doesn't entirely work with standard military practices, but it is very do-able. You do have a lot of positive things going for you, so we'll start with those. The fact that you mentioned that it's in a desert and that this is a fantasy world helps. A lot.

Good Stuff

Water supply

Assuming you have a proper river in your canyon, not some mostly-dead stream, you have a water supply that is very hard to mess with. Because...

  1. Once a river has dropped into a canyon, diverting it back out of the canyon is impossible. Any attempt to cut off your water supply would require them to travel all the way to wherever the river first drops into the canyon, then travel far enough up-stream that it won't just flow around your obstruction and drop back into the canyon anyway.
  2. Poisoning an entire river is HARD. You need a constant, high-volume inflow of toxins, because the toxins keep getting washed away and diluted by the massive volume of water that is moving through there. It's not like a well, where you can toss some corpses in and call it a day.

Historically, the city of Babylon was built straddling a river and took full advantage of the second point. It was taken after the Persians diverted the river and came in through the water grates. But as long as the canyon you're in is significantly longer than your city is...they're going to have a hard time plugging it up.

That said, you want to divert some of that river down into some unused tunnels to serve as a back-up cistern. People have turned islands into peninsulas in the name of capturing a castle...so don't assume your river is invincible.

Food

Using D&D as an example here...there are entire civilizations in your typical fantasy setting that live completely underground and grow all of their food in the complete absence of sunlight (see: The Entire Underdark).

If Dwarves can have vast cities that can feed themselves without ever seeing the sun, you can do the same. You really don't need to worry about running out of food at this point.

So, don't just rely on your terrace farms...go talk to them dwarves and get your hands on subterranean crops. Make use of all those tunnels you dug out looking for valuables, and grow food.

Stone Construction

The buildings of your city aren't flammable. This is good. The walkways and stuff (detailed far below) may be...but that's fine, they're supposed to get destroyed in a siege.

The Desert

You mentioned that Hurr resides within the desert of Durr. Deserts are phenomenal natural defenses. Little rain means little plantlife means little animal life...which means that an invading force is going to have a hard time finding food.

Additionally, if you in a hot (or cold) desert, the invading force has to worry about exposure to the elements. Heat Stroke will be a very real risk to them. Just getting an entire army to Hurr will be difficult, not to mention having to lug siege engines along with you (because there are no trees near Hurr apart from what may be growing down in the canyon). Thus, an invading army is only going to have what lives along the river as forage. They'll strip that bare pretty quickly. And, the bigger this desert is, the further they are going to have to travel to find provisions or ship them in.

Encirclement is nearly impossible

If this plateau/mountain is large enough to contain a massive canyon with a big city in it...it's huge. The enemy force is going to have a really hard time encircling the entire thing. If you have hidden passages that lead out of the mountain, you can probably come and go almost as you please. This continues to apply even if they take the top of the plateau.

The more easily you can slip out of your city without being caught, the more easily you can sneak around and set all their stuff on fire. Or sneak in provisions.

Bad Stuff

If they get on top the 'mountain,' your city is in trouble. And the mountain is BIG

Standard medieval military doctrine is that being on high ground is better. You can see further, shoot further, drop stuff on your foes, etc. Gravity is working for you when you have the high-ground.

Even if the canyon is in it's own little stand-alone plateau, you don't need to put a large army on top in order to cause some serious havoc. A double-line of archers firing down into the city would rapidly kill anyone exposed. And a tall canyon would result in the people on the floor of the canyon having an extremely hard time firing projectiles up at the top.

You're going to have to defend the entire mountain as if it were walls. If enemies reach the top, they can attack your gates from above, or just skip past the gates and start attacking your city. All they need to do is get a small team up the cliffs somewhere you don't catch them, and they are going to wreak all manner of havoc. (Kill the crew of a few guard towers to provide a 'blind spot' for more troops to scale the wall. Get behind the gates and open them. Dress like your people and set fire to farms and poison cisterns. And so on.) And if they can 'take' the top of the plateau, they're going to start getting siege engines up there to fire down into your city.

In short...your outer wall is vast and you have to defend the whole thing.

If you can field enough troops to properly man the cliffs...then this place becomes massively defensible...but this mountain sounds huge.

Effects of the above

Long-duration sieges favor the city

Any attempt to starve you out is going to go poorly. To starve out a fortification, the attackers need to have enough provisions to outlast the defenders. Because of Hurr's location within the desert of Durr (and the existence of fantasy-style dwarf crops), the city must be mostly self-sufficient, and can keep a good sized chunk of back-up farms safely in tunnels accessible from within the city.

So the attacker is sitting in a desert with a very limited ability to forage. They can forage along the river where it leaves your canyon, and that's about it. Meanwhile your city has spent the bulk of its existence living on what it can grow in the tunnels and along the river. They are not going to outlast you. Their only option is to assault the city.

Physical Defenses

  1. Set your gates back inside the mouth of the canyon a bit. This limits the angles from which your gates can be fired upon with siege weapons. Make sure there are towers either carved into the walls, or sitting on top of the edges of the canyon overlooking this little path leading to the front gate. If you can force enemies into a 3-sided death-box to assault your gate....they probably aren't taking that gate by storm. Make sure there are grates for water to get through...you'll need to defend these vigorously, as they are points of vulnerability.
  2. You need to treat the outer cliffs of the mountain this canyon is carved into like walls. Build towers, man them, kill anyone who tries to come up the the side. If you can have your diggers work to 'smooth' the outside of the mountain to make it harder to climb, that is also good.
  3. Routes from the lip of the canyon to the city should be destructible. If the enemy takes the plateau, you don't want them to be able to just walk their way down
  4. Build entrances to buildings above 'ground level.' Build ramps, walkways, bridges, etc. to provide access to and from the floor of the canyon. You have lots of money from the precious ores and gems you're harvesting...import that wood. Don't have too many of these, though...you don't want to give the enemy too much in the way of combustibles to target.
  5. Buildings should be interconnected inside the walls. Think fortress here...choke points are your friend. Try to tunnel under the river if you can dig deep enough, so the two sides of the city are connected. Choke points should be rigged so that you can cave them in if you're losing them.
  6. Build important stuff like archer posts in narrow parts of the canyon. Typical anti-wall siege weapons (catapults/trebuchets) fire in a parabolic arc. A narrow stretch of the canyon will interfere with this arc and prevent topside-based weapons from hitting your buildings without a lot of work
    • Additionally, put really important stuff deeper inside the walls where outside projectiles can't hit.
  7. Air vents, Air vents, Air vents. You have dwarvish-level digging tech: drill lots of tubes leading to random spots along the walls of the canyon so that smoke can get out and fresh air can get in if they collapse your entrances. Or, y'know...build fires in your entrances. Which is generally the best way to wipe out everything hiding in a cave
  8. Route parts of the river into underground reservoirs. In case someone takes the canyon floor, you want a back-up supply of water.

Tactics

Your goal in defending this city is to draw this siege out as long as possible. The longer you make this drag on, the more enemies the desert will kill for you. Between the river and your fantasy-style dwarf farms, you're in good shape for a prolonged siege.

Your walls (both the ones you made, and the walls of the mountain) should be defended in typical siege-style. Archers, boiling oil, the works. Your enemy is going to have a hard time building siege engines in a desert, and undermining is probably not happening against the very limited wall-area they can target.

Any attempt to flood you out is limited by there being multiple water-exits from the canyon, and the fact that waiting for the entire canyon to fill with water would take a long time and, again, time favors the defender when you're in this situation.

However, in a serious siege, the enemy is likely going to take the top of your mountain/plateau thing. It's just too big to fully defend, and someone is going to get troops past your defenses.

Frankly...that's fine. Pull all your forces back inside the buildings and tunnels of the city. Do NOT under any circumstances attempt to defend the floor of the canyon. You've lost your 'walls;' any attempt to hold there will be a slaughter. On your way in, destroy the access ramps coming down from the cliffs and up from the floor.

Now....the enemy gets to die in large numbers. In order to assault the buildings of your canyon city, they have to march into the canyon to assault the actual buildings. Sure, they can bombard you with any siege weapons they get up on top of the lip...but their targets will be limited by parabolic arcs and the difficulty of getting heavy siege equipment up on top of the mountain. Naturally, you should try to set those on fire whenever possible.

Ultimately, to take a city, you have to take the buildings and deal with the people. And you guys are holed up with your supplementary dwarf-style food production and entire caves full of water. And any time their army comes down into the canyon, they are walking into a narrow space with enemies possessing the high-ground on both sides of them. Rain down arrowy death.

If they do get inside, then we're on to tunnel warfare. Make good use of those choke points. Just plug em with corpses. Their best option for getting you out is to smoke you out...but that's what all the air-vents are for. They're going to have to go climbing around the walls of the canyon (likely while under fire from the other side of the canyon) trying to plug all the holes, or try to hit them all with siege engines and hope they collapse them, rather than just take a few layers off the outside.

Summary

Trying to defend such a place in real life would be hard...because city-supporting underground crops don't exist, and the middle ages didn't have dwarven levels of digging technology (such as being able to bore breathing hole tunnels). IRL, once you take the top of the mountain...battle is over. They just cave in your entrances and let you starve of suffocate. Or they lob big piles of burning stuff at your entrances, smoke you all out, and either just let you burn, or pick you off when you come running out.

A Plateau is wonderfully defensible if it is relatively small. Masada was, historically, one of the most invincible fortresses of all time. In no small part because it was built atop a plateau. But, said plateau was only 550m x 270m. Mount Conner is 3km by 1.2km...a vastly larger perimeter to defend. And to have such a canyon system with a full city inside it...yours is probably even bigger. Your enemy is going to have a hard time getting up there, but once they do your outer defenses are toast. And so, in terms of tactics...you plan for that. Try to keep them from taking the mountaintop, of course, but design the city such that trying to actually invade it forces them into a killbox.

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    $\begingroup$ As for polluting the river even if it is fast-flowing will require only an army. That army just have to use the river as their toilet. Cholera and other nasty things will follow. $\endgroup$ – BentNielsen Jan 23 '18 at 19:49
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The city is entirely built into the "walls" of a canyon (pretty much like the Great Canyon) which has a flowing river on the bottom.

I presume you mean the Grand Canyon ?

Most of the buildings are on the side of the "cracks", but some can be constructed on the flat top. Suppose there are a few entrances at ground level (e.g. where the river gets larger and enters the plains outside).

There are four issues here :

  • The top will be the location of attacks and as you've effectively lost the high ground, which is a form of military suicide, you would need very strong defenses on top. But even these can be breached - static defenses are doomed in this sense as eventually any static defense can be breached.

  • If you station troops and fortifications on the top, you cannot give them direct access to the city itself. To do so would be to introduce weak spots, making the fortifications themselves locations ideal for attack.

  • Access to the river is very problematic. It allows your attacker two potential vectors (top and bottom) all along the city. This requires you to defend both top and bottom, splitting your forces.

  • They have mobility, you don't. Your defenses are limited by the construction of your external "facade". How many resources and men you can deploy is therefore limited. As your internal movements will be limited by corridor sizes and the need to move vertically by stairs ( a choke point for movement ) you have more difficulty that the attackers do in moving to a new point to enter.

Few to no buildings are built on the bottom, most of those being infrastructure related to water (or walls and gates, in the outermost parts of the city).

They can attempt to undermine you on the bottom. This is exactly what it sounds like : they mine under your existing systems, perhaps by developing a beachhead at one the existing entrances. They can then tunnel and use that to deliberately undermine the structural integrity of the support for the levels above. This is a well established technique in ancient warfare.

It becomes even more problematic for you if they can deploy any form of explosives (including magic).

Food is gathered via hanging gardens and terraced farms inside the city.

Unless you have some very strange things happening (magic ??) you have gardens indoors without sunlight to grow things. That's a major problem. You can deploy these areas to the canyon walls (which will probably be in shade anyway) as they'll be easy targets for the enemy.

As you're stuck inside you are in a siege situation. You cannot forage for food outside and must rely on stores inside.

The city came to be because of the humongous quantity of valuable gems and materials present underground in the area. Obviously, this makes it a target for attacks.

If you have underground gems you have mines protected by fortifications. These are much simpler to defend in more conventional ways.

The city is a source, perhaps, of manpower, but in a sense the manpower is irrelevant. You need to protect the primary asset and by keeping the manpower away from it in conventional towns you would create a potential militia force to help defend any mines, as well as themselves.

One the manpower is trapped in your essentially underground city they loose any mobility to respond to attacks and become a burden to the defenders (e.g. your children are not a particularly useful fighting force, but they do require food and drink and protection).

So how could such a city effectively defend itself from sieges, with medieval-fantasy technology? (some magic is acceptable, but I don't want stuff like "a wizard makes the city invisible to the outside")

Sieges against conventional and even highly fortified cities have been conducted throughout history and at best the defenders outlast the attackers, but typically at the cost of massive death by famine, death and disease. At worst (which is perhaps a better than 50% chance) the city defenses are breached and the entire population massacred. As you've given up the high ground your population already has a major problem for defense.

I'd consider this a death trap, and if I was attacking I'd feel right pleased with the prospects for my endeavors.

If I was defending I get the heck out as fast as possible - desertion is OK in my book and beats certain death by slow starvation any day. :-)

Technology level is comparable to that of D&D dwarfs, so something a little steampunk-ish is acceptable (e.g. they probably have huge dams to control the flow of water, and a good hydraulic system)

I don't think "steampunk-ish" makes much difference. We've had major sieges of strong underground fortifications before and the attackers won. Think of the complex and actually well designed forts of the Maginot Line. These were successfully attacked and contrary to popular belief they were breached.

The lesson from these systems is that even a well designed system with rapid internal transport systems can be breached and that once there is a breach along the line, you effectively split the defenders off into two smaller groups who cannot aid each other or seal the breach. This is another reason I'd be happier attacking than defending.

And dams ? Not for long they wouldn't. What attacking army is going to leave you in control of those ? So those systems would probably become either useless to you or, worse, an actual weapon for the enemy.

And dams imply flooding is possible, another significant vulnerability. You need underground rivers and reservoirs, not a dam.

What are the biggest threats? (I'm assuming water shortage and an "attack from above", although I'd say the latter would be pretty impractical)

Attack from above is fabulous !

You have gravity working for you. In conventional sieges of castles and fortified walls you have to pass through a gauntlet of fire coming from above - stones, hot oil, any form of projectile. It's much harder to shoot up at them (generally requiring siege engines or the construction of elevated platforms to fire back from). And the attacker has to scale the walls somehow (or break them).

All your attackers have to do is fall, maybe nothing more than a jump or descent by rope - fast, easy to reinforce, hard to prevent. Foot holds (breaches) are relatively easy to reinforce for the attacker, and relatively hard to retake for the defender. Defense after an (inevitable) breach would be a matter of using choke points and trying to prevent further breaches that make defense harder. As breaches can be multi-level your defenders cannot restrict what they have to defend to just e.g. a single level on top of a high wall. You have to be ready and able to tackle breaches everywhere !

No, I'll be the attacking general on this one, thanks.

Also keep in mind that, as any siege progresses and food and conditions worsen, your city population may decide that they've had enough and would prefer life under the heal of the oppressor than e.g. no life at all. You can have a revolt internally and they could create a breach that the enemy can exploit.

So in a sense you would be trapped and besieged both within and without.

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  • $\begingroup$ Well to clarify the "attack from above", [...] would be pretty impractical) part: I actually gave the wrong example citing the Grand Canyon. The entrances to the "bottom level" are on ground level, and the top level could be seen as mountains. Sorry, I don't have an accurate real-world example (maybe something similar to the mountains in Australia?) $\endgroup$ – Hankrecords Jan 22 '18 at 18:46
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    $\begingroup$ @Hankrecords Unless the mountains are tall enough and treacherous enough to be impassable to any sort of large force, you still have the same problem. As long as an army can reasonably get up on the lip of the canyon...defending a canyon against siege is going to suck. $\endgroup$ – guildsbounty Jan 22 '18 at 18:56
  • $\begingroup$ @guildsbounty I updated the question to make it hopefully clearer $\endgroup$ – Hankrecords Jan 22 '18 at 19:59
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When our great fathers and great mothers first built this city it wasn't to escape violent men. It was to escape the storms. Violent storms sweeping the sand along so fast that it will leave you blistered or worse. I've even heard of the winds flinging small stones through a horses head. They happen frequently, several times a month.

At first the settlements in the canyon were small and spread out. But then we discovered the Red Beryls and our population grew. Our fathers and mothers where wise. They knew that the storms that drove us here would keep us safe from above. But we were still vulnerable to siege. So they stored large amounts of grain. in case our enemies blocked up the canyon entrance by the plains. We have listening post deep in the canyon wall were we mount large metal shields on the wall, so that if our enemies try to dig their way in, the vibrations will make the shields ring, and we will know where they are coming from bellow.

Once before when I was just a lad we opened all our locks and dams releasing our all the water stored in our small lakes and ponds at once. We flooded our enemies away from the canyon mouth. But alas my people are growing lazy. Too much prosperity for so long has led to greed, and carelessness. Some worry about corruption in the city guard, and I've heard rumors of rats in our grain stores. The council of elders argues day by day about the most trivial of matters, and I worry that our enemies will find a way to break us from with in.

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  • $\begingroup$ I love the style of your answer! :) $\endgroup$ – Hankrecords Jan 22 '18 at 20:25
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The weak point is the flat top

I think that the weakest point is the flat top. If there were no buildings, nothing would prevent the assailants from trying to bore holes straight down. Then they could either go into the town, or drop bombs (incendiary bombs, oil lamps), or pump poisoning gases (even just smoke would do), or simply weaken the structure until everything collapses on the defenders.

In a non-medieval yet steampunk-ish tech setting the assailants could use Thermite, and as long as they can provide fuel from above, it would continue to descend by mere gravity force. In a medieval setting, they could still try to smelt the rocks, one fireball at a time, or they could just manually dig. The Romans were quite good at digging under thick walls to make them collapse, and that was long before the middle ages.

Coping with the possibility of disaster

To prevent this disaster, they need fortifications on the flat top, or they need to make it inaccessible, for instance, if the town contained an island in the canyon, such that access was not granted unless someone would go through the city. Another possibility is to have pressurized steam running between the rock sheets of the flat top and the ceiling of the city. When a holes are dug, jets of overheated steam will come out, effectively preventing further access to what lies below.

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Attacking from the top is not practical

How do you attack a stone wall with medieval or earlier technology? The answer is that you mine the wall. You dig under it, causing it to collapse from its own weight.

Your city is basically the wall, itself. Given the picture above, some of the structures appear to be carved out of the solid rock a la south Indian rock temples:

enter image description here

or Petra in Jordan

enter image description here

How are you going to mine these pictured building? You are not. How else can you get in, other than from the front door? Well, you can't really dig from the top; that is solid rock and you'd be tunneling for months if not years.

Basically, I argue that if the defensible buildings are built into the rockfaces, there is no good way to mine or tunnel into the building, because of the strength and thickness of the rocks involved. The most famous carving at Petra is the Treasury, which is about 40 meters high. The Grand Canyon has a depth up to 1800 meters. If you start 50 meters above the canyon floor and build up 250 meters (almost the height of the Empire State Building), then you still have just short of a mile of solid rock above you in the Grand Canyon. How are you going to dig through that?

Catapults and trebuchets were around in this time period, but they threw relatively small rocks and weren't very useful for battering down walls. They were more useful for attempting to destroy emplacements on those walls in preparation for an assault. The rocks thrown by these machines were too small to do serious damage to a city built into a cliff. Now, decorative elements could be destroyed, but you never conquered a city by making it look ugly.

Finally, there is the option of throwing boulders down onto the city from above. There are two difficulties here; first is that hitting anything is not trivial. Second is that the walls and city are one. In destroying the defensive fortifications you also destroy the city. The whole purpose of taking the city is to plunder its loot, or subject it to tribute. If you reduce the city to rubble, that isn't going to help your wealth generation. I argue that not only is hitting this city's vertical walls with a boulder dropped vertically hard, but also counterproductive.

Conclusion

The city is extremely defensible. There is no easy access from below, no easy access from above, and it can be too high to scale with ladders. The walls are to strong to hit with catapults, and vertically built into a cliff-face they are too hard to hit with boulders, if you even wanted to do that.

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    $\begingroup$ I feel this is more true of cities built into mountains and cliffs...rather than a city built in a canyon. If there's plenty of flat-space on top of the canyon, then it would be easy to put an army up there and just start dropping stuff on the city. $\endgroup$ – guildsbounty Jan 22 '18 at 19:23
  • $\begingroup$ @guildsbounty I think you are seriously underestimating the difficulty of accurately dropping on a target half a kilometer below you. If you threw a tennis ball off a skyscraper, how many tries would it take to hit your target? Now imagine that your target is on the side of an adjacent skyscraper, hundreds of meters below you. Not so easy. $\endgroup$ – kingledion Jan 22 '18 at 20:36
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    $\begingroup$ You don't try for 'accuracy' in this case, you go for volume. Drop lots of stuff. Shoot volleys of arrows. Drop jugs of burning pitch. Pour boiling oil. Same thing battlefield archers always do. You don't aim for a specific target, you aim at a general area...then flood it with so many projectiles that you're going to hit something. $\endgroup$ – guildsbounty Jan 22 '18 at 21:10
  • $\begingroup$ @guildsbounty I'm sorry, but have you heard of doors? What is a jug of oil or a few arrows going to do against foot thick doors? $\endgroup$ – kingledion Jan 22 '18 at 21:36
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    $\begingroup$ That's what the burning pitch is for. Set doors on fire (if they are wood), or fill the area with smoke to start choking out the occupants. Also...we're talking about a canyon here....you can go stand on the other side of the canyon then start lobbing stuff via catapult in on nice parabolic arcs. A narrow canyon would be a good defense against this, but provides an even tighter area that will fill with smoke faster. $\endgroup$ – guildsbounty Jan 22 '18 at 21:45
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I believe others have given good arguments why your city is unlikely to survive a direct military conflict without unsustainable defenses or magic.

For a solution to a siege I'd direct you to Asimov's solution in the Foundation Series.

The beginning of this story features a militarily indefensible, yet technologically advanced planet surrounded by 5 warring kingdoms, any one of which could easily annex the protagonists if they so chose.

The solution offered is for the planet to become extremely rich in exports of technology, to the benefit of all factions. No faction is able to seize the planet without being immediately destroyed by the other four, as none have the capacity to manufacture themselves, and losing the technologies on offer to a single enemy would be military suicide.

In that story the technology was nuclear power. The makeup of your city could allow for these to be the potentially magical or valuable nature of the gems and metals native to Durr, or the advanced and secretive industrial techniques your city developed to sustain itself in such an inhospitable setup.

So long as the political situation in your fantasy realm remains relatively stable, Durr may not need to beat a siege. They can survive with efficient, unsustainable defenses (such as walling off the few entrances pictured) with scouts for early-warning, secure that other self-interested communities will break the siege themselves.

A status quo like this may be doomed to eventually collapse, but that itself poses interesting narratives for your world.

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  • $\begingroup$ I really like this idea, I hadn't thought about this solution! It could be a Switzerland squared: completely neutral, and in everyone's interest to defend from others because of important resources. I'm afraid I can't mark this as the answer (since I explicitly asked about defending against a siege, and while your answer solves my problem perfectly I think it wouldn't be fair towards others who perhaps didn't suggest this just because I asked something different). Giant +1, though :) $\endgroup$ – Hankrecords Jan 23 '18 at 10:19
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Defend forward

Your first advantage is height. Put sentry posts checking for possible enemy forces. If anything suspicious is seen, send your army to block their possible upward routes. With any luck they will be narrow enough to allow to face your enemy piecemeal, with the top position advantage. If the paths available are too wide, just fortify them.

Even if you fail to defend the tops, making difficult its access will stop the arrival of supplies to the enemy, which is your long term goal.

Starve your enemy

If the previous point fails, fall back to the wall you have already built around the canyon. Here you have:

  • A disavantage, in that the design of your city is too lineal. That means that your walls will be relatively long and you may be lacking manpower to defend all of them. Good interior lines (to move reinforcements to wherever they are needed is must.

  • An advantage, as the enemy army will have trouble supplying themselves. No food grows on rocks, and water is down the canyon. Ensure that you do not need food stores near the enemy reach. Try to defend as much of the river as possible. If possible and needed, don't be shy to be ready to dam it to prevent your enemy access to water. If people dies, throw their bodies downriver towards the sections of the river they control.

Defend your citadel

Create a safe area, easy to defend.

Water supply. Lots of room for people, food and other supplies. If everything else fails, burn anything that you cannot take with you and lock yourself in.

Make entry as difficult as possible, exit as easy as you can without compromising entry. Wait for the enemy to discover that there are no supplies to plunder and that, if they want to get you, maybe they will run out of food first.

Sneak out

It is your home. You know every bend in the canyon, every cave. Which caves do have an alternative exit elsewhere in the zone. Or just dig some of these caves until you get alternative exits. Ensure that the entries have chokepoints, so if the enemy discovers them they can easily be defended.

Keep the entries hidden, and guarded. Use them at night, to avoid the enemy detecting them. Use them to launch attacks against your enemy back, to smuggle supplies, to communicate with your allies.

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Desert crops are are vulnerable to fire

This is a fairly crude answer, but a simple approach would be to get a few hundred archers to shoot flaming arrows on the crops for a while, and once they're torched, you could conquer the cities around Hurr on which it would depend for food, and once you'd done that, Hurr would have no choice but to surrender or starve. This is assuming a foreign army can burn the crops, but they're in a dry environment, and fire is contagious, so I don't really see an issue with that part of it. I doubt Hurr's irrigation infrastructure would be sufficient for dousing the fires fast enough. Even if the archers can't reach the top of the mountain, just one raid (where you accept you'll lose everyone who takes part in it) to get the fire started would be sufficient. Especially if the raid can knock down some irrigation infrastructure too. This lowers the bar from invading them successfully to sneaking a few guys past their defense once.

The only defense for this is either trying to keep reservoirs on top of the mountain for dousing (which would be hard, because it's in a desert), or not to let the invaders get close to the city, which is so difficult that it can barely be considered a defense.

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To prevent attacks from on top change the outward appearance to something more like mount roraima this sits between Brazil, Venezula and Guyana.

enter image description here

There is only a single passageway up to the top and it's a full day trek and it's only wide enough for a single file line, it's hard to even cross another team coming up from the bottom. Also there is a water fall on the path, so if it's rained a bit in the last hour you have to wait.

This would then mean that the only way in would be the river. This is much more defensible for the people.

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A mesa, such as Mount Conner would be superbly defensible. The cliffs would be cut to make scaling them only possible by skilled rock climbers. On the top, I would have an array of ranged weapons to keep an attacking army as far as possible from the cliffs.

As an attacker, the best option would be to encircle the mesa and starve you out. Even if you have sufficient crops to feed the entire population, you need water for irrigation. For that reason as attacker, you would divert the river, so it no longer flows through the mesa, but around it. Even if with underground water storage, that is going to run out eventually. Unless you can get water, the attacker eventually wins.

The solution: An underground aqueduct. The aqueduct would connect with the river as far as feasibly possible away from the mesa. The underground storage would be constantly refilled from the aqueduct. With luck, you can outlast them.

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enter image description here

If the place looks like this then the attacking army is coming from outside, right? So unless they can fly they shouldn't be able to get in. They might have massive catapults that they can use to throw stuff over, but if the walls are high enough then that might not work either, not to mention that they couldn't see what they're hitting. Most of their shots could hit nothing.

So they only have entrances/exits to worry about. If the conflicts center around those, the people on the inside just have to wait out the attackers.

Whats that you say? Shouldn't it be the other way around? Wouldn't the attacking army just have to wait out the ones on the inside until they starve? Not if they have their own farms and livestock in there. It certainly looks big enough that they could do it. And they could have large caches of water too. Sure those would drain away sooner or later, but not so soon that they couldn't wait out the attackers. And there's rain to replenish that (I know its an arid place, but if they can trap the water one strong rainfall could last a long time), or wells, or an underground water source. Mountain climbing enemies could be kicked off the cliff side or squashed with avalanches or burning oils.

In a sense the attackers are the ones trapped outside in an inhospitable desert. And the people inside could use their catapults to much greater advantage as they have the high ground.

If the canyon city is self-sufficient they could hold out much longer than an attacking army might. Think the Fremen of Dune. Or even Masada, but better (and with a happier ending, no offense intended).

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