Say there is a floating city that rests high in the air, it has the area of around New York City and has massive hydrogen tanks under it to keep it afloat, (I am aware the physics of this is impossible, but this takes place in a fantasy setting so I am not concerned), and the residences have air filters to counteract the altitude. Now say that this city is at war with another city, and one of them launches an attack. How would this go down, more specifically, how would this enemy city besiege another? This would not just mean destroying the tanks under the city as it is necessary for my story that these people board it and rescue a person inside the main palace whilst doing so.

City Structure This city rests on a platform of thick concrete in a perfect circle, and as I said has massive tanks of hydrogen keeping it afloat. The building as made of wood as to not weigh down the structure. There is a main structure which is a sort of palace located in the middle, however, it is highly defended. The city also stays in one spot and does not drift. This city grows crops inside the city walls as well as harvests condensed water from clouds, so cutting off supplies is not an option.

City Defences See question:Siege defences for a floating city

General airship design These airships have the hull of a regular 12th-century medieval liner. This liner has two leather horizontal gliders on either side (outriggers) responsible for keeping the airship afloat. These airships can also go above and below, as they can leak and add hydrogen to the tanks. (Note that you can change design structure as long as you ask in the comments and I approve first).

Notes Has technology equivalent to that of Imperial China. Is follow-up to:Siege defences for a floating city

  • $\begingroup$ What is a 12th century medieval "liner"? $\endgroup$
    – Slarty
    Oct 29, 2017 at 21:06
  • $\begingroup$ @Slarty Just think of your average medieval ship, yeah, that. $\endgroup$ Oct 29, 2017 at 21:08
  • $\begingroup$ What use is the sail? The ship will move with the wind anyway, since there is nothing to prevent its movement. $\endgroup$
    – AlexP
    Oct 29, 2017 at 21:09
  • $\begingroup$ @AlexP The outriggers keep it afloat and the sail in the middle is for steering. $\endgroup$ Oct 29, 2017 at 21:13
  • $\begingroup$ Key question why 10,000 feet? Or more importantly can the vessels fly higher than the cities? This is important in determining the siege options. The word “siege” implies some form of impenetrable or near impenetrable barrier what barrier do the cities have? Is it just their height? Also how do the cities get their supplies? $\endgroup$
    – Slarty
    Oct 29, 2017 at 21:14

8 Answers 8


If the airships can get above the city it’s more a matter of airborne invasion than siege. There is the possibility of making an opposed landing on the city itself. Paratrooper drops would also be an option and even gliders if they wanted a silent approach at night.

Actions that can be taken in defence would be: balloon barrage – as they have the city they should be able to build a much smaller version and tie a rope on it. Netting to prevent the entry of enemy soldiers and of course lots and lots of archers and catapult artillery.

If the airships can’t get as high as the city then it’s a proper siege and could work by preventing resupply via air ship. Presumably there is some method for the airships to unload – either by docking at the very lowest point under the city or with a platform that could be lowered.


Flip it.

flipped pontoon boat from source

Your city is balanced atop its floats. It has two stable positions, as anything on floats: floats under city and city under floats. If you can flip it upside-down it will stay that way and it will be tricky for the occupants to mount much of a defense: if lucky they will be on the ceilings of their buildings, and if unlucky between their roofs and the ground.

Your besiegers would attempt to attach winch ropes to buildings or other structures and then use these to winch the floating city up and over. The ropes might be attached to the ground, weight ships that open their float tanks to become heavy, or powered tugboat-type ships that use brute force to pull the thing over. One might supplement this endeavor by lifting the opposite side of the beseiged city: a heavily ballasted stack ship drops ballast while under the city and comes up below it.

The point of this sort of attack, as opposed to just dropping bombs from a height or blowing up the float tanks, is that you can capture much of the city intact. The loot and tech will mostly be inside. Once flipped back, the city may need some repairs but just like a boat that has been overturned, the structure will be fundamentally sound.

It would be cool if the engineers of a floating structure anticipated this sort of problem and built to some degree to operate upside-down as well.

  • $\begingroup$ How do you flip something that is miles across? This is akin to trying to flip one of these monster icebergs that break off Greenland or Antarctica $\endgroup$ Oct 29, 2017 at 22:08
  • $\begingroup$ That would be cool. Hmm...... $\endgroup$ Oct 30, 2017 at 0:17
  • $\begingroup$ Capsizing a catamaran is really hard compared to capsizing a single hull vessel of equivalent size. However these aren't displacement hulls like ships and can't be compared to a surface vessel. There's no displacement difference in the stability no matter the angle, a submarine is a better comparison. $\endgroup$
    – Separatrix
    Oct 30, 2017 at 15:26
  • 2
    $\begingroup$ bursting some of the side tanks would help this a lot without sending the whole city plummeting. $\endgroup$
    – anon
    Oct 30, 2017 at 15:45

In air warfare the general rule of thumb is whoever has the elevation advantage is the victor. If the besiegers have won the ship to ship engagement and control the skies above the city, they will slowly bombard it into submission.

Rocks will work, but the best way is fire. Burn out their crops, force them to use all their water reserves to put out structural fires until all their water is gone. If they do not fight the fires then their buildings are destroyed and they must survive in the open.

Reinforced concrete bunkers? Just keep dropping stones from high enough and you’ll get through eventually.

When everyone is hiding in their bunkers you could then stage your rescue assault. In general though, in a world like this, I feel a city will generally surrender once their fleet is lost, unless they expect a rescue/relief fleet to arrive within 3-7 days.


If you want a romantic version of a siege make the attackers' plan be to force it to the ground.

This might not be militarily sound, but to people who live in floating cities it might be a particularly humiliating defeat with little loss of life or infrastructure.

If you add mass to a lighter than air craft it will eventually not be lighter than air and will fall. The fall speed would be related to how much heavier than air it becomes. With a city a million tons will probably not be enough to make it crash, but if attackers are consistently able to add mass faster than defenders can remove it it will eventually get to the ground.

You could just drop rocks on them. It is a lot of hassle to move big rocks while big rocks are being dropped on you. It would probably be a lot of rocks, but exactly how silly this notion is depends on how much more lift could be made than the city currently uses. Chances are a city would tend to grow to within a fairly narrow safety factor of maximum lift. So you might not have to do nearly as much work as was done to make the city.

If the attackers could foul the sewer system water weight could be a big help. If the city is dependent on rain it probably has cisterns. Fouling them with something hard to clean like a sludge-making algae makes them dead weight while the city still needs to store about the same amount of water.

Foul the lifting gas. You can't burn it or release it because you can't control that, but if you add a limited amount of something that reacts with the hydrogen you might leech it out in manageable amounts to let the city come down slowly. If you can catalyze a hydrocarbon or ammonia (not known with reasonable pressures or temperatures) you could do it with only a poor mass ratio, but I understand some metals like palladium bind to hydrogen readily.

If you want a fun siege use tactics not possible in the real world.

Snatching buildings. The attackers drop hooks or special forces to secure lines to a building and cut its foundation. Then big lifting airships pull the building off the city. The building can them be separately besieged, shaken or dropped as the attackers will, bad times for defenders.

Divide and Conquer. Drop walls or fences onto the city streets, this limits the defenders' ability to move between parts of the city, but the attackers come from the air and aren't bothered.

It a tarp! Build a giant tarp and hang it above the city. Without light or rain they will have problems. Drop it on them full of water or on fire and they will have serious problems.

Really the fight is over as soon as attackers have air superiority.

The defenders might be able to make a decent peace if city killing is frowned on and they hold a valuable hostage, but the end is certain.

At the very least attackers can deny water, if they have a technique to harvest clouds directly attackers can do it first. Every cloud coming near is intercepted and drained before it reaches the defenders' collectors.

Even without gimmicks the ability to lower troops anywhere and support them with arrows or rocks is insurmountable.


Sails won't work.

Sails work with with ships because the water is (relatively) motionless. This allows the force of the sail to be split into two vectors, one pushing the boat forward, the other attempting to push it sideways. The hull and keel is designed to make the first a lot easier than the second. But in navagating you will see references to 'course made good' because of the sideways drift.

In the days of wooden boats and iron men, a ship of the line had a tough time doing an upwind course within 45 degrees of the wind. That is for a west wind the best it could do was either northwest or southwest.

In an airship, the only direction a sail will take it is down wind.

You may be able to give them air screws. See Niven & Gerrard's novel "The Flying Sorcerers" or import Tarns from Gor, Dragons from Pern, or Roc's from the Adventures of Sinbad.


Do you have powerful enough motors? You might want to look into thrust-to-weight ratio. At least you would need a hydrogen/helium source and a lot of unskilled, but relatively fine labor.

All designs with dirigibles and coal engines failed. A hard-structure dirigible (a zeppelin, in contrast to a blimp) and internal combustion engines work, but that's 20th century for you. But also any heavier-than-air aircraft is.

If your siege forces can procure enough silk and hydrogen or helium, they might use blimps to obscure the line of sight and movement of auxiliary vehicles of the flying island. That's basically an AA technique used in the Second World War. Of course, if the siege forces have their own aircraft they can employ cruiser warfare on the supply chain.


A traditional siege will entirely depend on whether your attacking force has effective air-to-air combat capabilities, and the self-sustainability of your city. Sieges are generally based on cutting off supply to a city until it falls due to lack of food etc. This could be done pre-air by building a wall around a city, and preventing anyone going in/out. These days, building a wall will not work, due to airplanes. So it requires preventing resupply by land, but by air also, which means you need effective anti-air capabilities (either AA guns, or fighters to patrol). This is really only possible I think due to radar, as otherwise catching all the planes will be impossible.

So in your scenario, are the attackers capable of taking out enemy airships coming for resupply? Is the city really self-sufficient in terms of water, food, etc?

Your parallel question refers to retrieving one person. If the attacking force is trying to catch an unwilling prisoner, then as noted on that question, it will be very difficult for them to find the person in a city that size without a full-on invasion. If that person is trying to get out, then it's very easy – they just have to find a hole or the edge, and jump, and use a parachute / wingsuit / dramatic-mid-air-landing-on-an-airship etc. to escape. It will be virtually impossible for the attacking force to prevent one person escaping, spotting a single person falling from a city that size at night would require massive surveillance, e.g. radar (or the magical equivalent), and then catching them would be problematic, assuming they have someone outside with an air ship to help them – or even a fast horse ready.

  • $\begingroup$ Yes, but the person does not have any wing suit or anything. And it impossible to escape without any kind of assistance. $\endgroup$ Oct 30, 2017 at 16:31
  • $\begingroup$ OK, don't know what situation your 'rescuee' is in. However, the same issues apply to someone dropping into the city as escaping, so an agent or two could easily drop from an enemy airship into the city, and – assuming they can find the person – escape with them. If the person is in a high-security jail etc., then obviously that won't work. $\endgroup$
    – Dan W
    Oct 31, 2017 at 17:23
  • $\begingroup$ If your 'rescuee' can only be released by putting enough pressure on the government, then you'll have to rely on the siege, and as I said, that'll depend on the city needing something from outside (but you've made it pretty self-sufficent). $\endgroup$
    – Dan W
    Oct 31, 2017 at 17:24

Siege Blockade

One typical strategy of siege is simply blockade of a city.

Cities have an inherent weakness in that they require a constant flow of resources in in order to support. This is especially true for a floating city, where do they get the water food and resources to support all the defenders necessary to stop a fleet of invaders.

In many cases sieges end up being a waiting game waiting for the defenders to dies of disease and starvation.

Also, there is no way the city could grow all the food as well. Human diets are incredibly complex requiring different sources of nutrition. (Scurvy Rickets)


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .