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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 residence 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 the city defend against a siege, considering the attacker’s intentions are to board and retrieve a person from the palace?

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's are 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. This city cannot drift and stays in place. Also, they grow crops inside the city.

Attacking airships These airships have the hull of a regular 12-century medieval liner. This liner has two leather horizontal gliders on either side (outriggers) responsible for keeping the airship afloat, as well as a sail above the main vessel for propulsion. These airships can also go above and below, as they can leak and ad hydrogen to the tanks. For siege equipment see the question:Siege technology for floating city

Notes City’s people have technology equivalent to that of Imperial China. The airships could not just destroy the hydrogen tanks under the city as that would kill everyone inside, including the target. Is a follow-up to:Siege technology for floating city

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  • $\begingroup$ Sometimes killing everyone inside, including the target, is precisely the objective. It is a political decision. If the politics of your world make that unlikely, say so in your question. $\endgroup$ – user535733 Oct 29 '17 at 21:45
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    $\begingroup$ The land area of New York City is over 780 km^2. Your attackers, even with tens of thousands of troops, are simply not going to locate a warned target without a summoning charm to make the target come to them. With a summoning charm, they need not besiege the city at all....since the target will come to them. $\endgroup$ – user535733 Oct 29 '17 at 21:51
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    $\begingroup$ Recovering a spy or agent does not usually require a siege. Seems more of a covert action. Why should (potentially) thousands of soldiers die for whoever sent this distress call? $\endgroup$ – user535733 Oct 29 '17 at 22:12
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    $\begingroup$ You specified 'siege' - a particularly brutal form of warfare, deliberately starving the city to weaken the defenses. The romantic technologies and strategies to defend the city seem largely irrelevant - anti-siege strategies must be centered around maintaining an adequate external food and water supply to the city. Is that really what you are seeking here? $\endgroup$ – user535733 Oct 30 '17 at 1:45
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    $\begingroup$ Forget the nay-sayers. I love this plot. Very Jules Vernish. Imaginative, but not outrageously wizards and potents magical fantasy. $\endgroup$ – Justin Thyme Oct 30 '17 at 4:31
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A floating city. Floating ships. Interesting.

If they have the technology for concrete, and for hydrogen flotation, I suspect they will have the technology for hydrogen balloons and some chemical knowledge. Britain used a similar trick in the second world war. Set up a flotilla wall of hydrogen filled balloons, with igniters (something like glass jars within a jar that separate sodium in water from air). If the attacking ship hits the glass jar, the jar breaks, the sodium ignites, the hydrogen explodes, the attacking ship is, well, toast.

If your side has more balloons than the other side has ships, you can break the siege.

Also, if they have the technology, burning arrows shot at the hydrogen outriggers of the enemy ship. BFB. (The B's are Big and Boom). Maybe the archers are in hot air balloons.

If you can summon lightning, even better.

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    $\begingroup$ So basically skymines? $\endgroup$ – JAD Oct 30 '17 at 12:25
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You mention that the attackers can't destroy the hydrogen bags because that would bring down the city, killing the agent they wish to recover. Therefore, the attack itself must be limited. You can't hit the palace as that would kill your agent, too. Therefore...

Attack from Below

You want to disable the city to minimize its defence. A little tilt doesn't hurt anyone, but does cause confusion with defences. Therefore, you can't rule out a piercing bag attack.

  • Shielding on the side, like a thick sheaf of bambo, so that attacks from the side or above cannot impale the hydrogen bags.

  • "Wine glass stems" are what I'm calling bamboo constructs that hang by a stem between hydrogen bags with a large, flat circle at the bottom. These deflect attack against the bags from below. Make them either releasable or the stems easy to break such that people won't be tempted to climb on the wine glass stems to board the city. Or, make them strong enough to act as platforms for invading forces. And I really like that last idea because...

  • The easiest way to board the city is to come up from below and grapple the concrete edge. You need a way to stop that, and my wine glass stem defensive platforms are just the ticket.

The Best Defense is a Good Offense

The fundamental problem with all airship-based stories is that the airships are trivial (per se) to bring down. Pierce the bag, drop the ship. You can armor an airship only so much due to the need for lift (unlike ground vehicles where you can distribute weight on the ground by adding more wheels, which are under the armor). Therefore...

  • Ballistas... a forest of them. Your perimeter should bristle with ballistas. Ballistas are great for long-range with, shall we say, up to a 50° inclination. (You could go more, but wind + high inclination means you're spearing your own people.) They're precise, but not necessarily machine guns. So, their purpose is to rid you of whole airships at a distance.

  • Greek fire, which is an early flame thrower (and so effective it's nearly mythological). This is for close-range attacks where precision isn't as important as coverage. While I like this idea, it's really only useful on the perimeter without threatening the city.

  • Focused mirror towers. Twenty feet in diameter, concave, one or two focusing lenses, and you have yourself a makeshift beam weapon capable of delivering the dragon's own judgement against your intruders! These lovely contraptions are forced to only attack into the sun unless you have mirrors throughout your city that direct the light to one or more towers that redirect and focus the light.

And lest we forget, the Chinese were and are masters of pyrotechnics

Finally, there's nothing more satisfying than shooting a bottle rocket into your incoming horde of hydrogen-bathed heathens. Hydrogen really is a whomping weakness, and a good nylon would stop a bottle rocket ... but not one of its larger cousins. Don't overlook the value of fireworks.

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    $\begingroup$ Attacks from below better be stealthed. Otherwise the defenders just drop stuff on them to thwart the attack. $\endgroup$ – PCSgtL Oct 31 '17 at 13:16
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Breaking the blockade
Interception of the blockading forces by friendly airships

Attack on the blockading airships from the ground Any enemy airship blockading in the area around the city but not under would be susceptible to attack from the ground. Assuming they can use the float technology on a small scale Ariel mines could be devised to lift nets, ropes, sharp objects and explosive or flammable materials up under the airships and ensnare them. This might be used under the city but at the risk that any “misses” would continue to rise and would probably hit the city, which might or might not be catastrophic depending on the lifting technology and how rugged it was.

Attack on the blockading airships from within the city Attack from the city might be particularly effective if the blockading ships were attempting to prevent blockade runners as described below as they would have to operate under the city. The size of the city becomes important as to how effective this would be. But as an example an enemy ship approaching underneath the city could be bombed with boulders, spiked balls explosive or flammable materials. These could be tied to ropes and nets and dropped in a pattern to intercept the movement of the vessel vaguely similar to the use of depth charges by destroyers. Some form of burning tar/oil might prove particularly effective if it was available in quantity and it was used to pour down on the blockading ships.

Running the blockade
Fast airships might be able to out run the blockading airships and gain access to a safe harbour of some sort (the exact layout and design of the airships docking with the city is an important unknown here). An even better way would be to use small scale lifting bodies to rush supplies directly up underneath the city. Such supply runs could be protected by city attack downwards around the central resupply point and ground attack upwards beyond the boundary of the city.

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Bear in mind that the lift of hydrogen is a bit over a kg per cubic meter. Some playing with numbers says that the tanks are going to be the tail that wags the dog. Float the city by magic instead. It's far more believable.

Winds will push the city sideways. How to you prevent this?

Under normal conditions how to people get on and off the city?

Where does the water come from?

Where does the sewage go?

Where does food come from?

What is the advantage of a floating city?

How high up is it floating?

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  • $\begingroup$ The city is connected to a long rope to the ground, so there is no sideways blow, however cutting this rope leads to the emediate killing of the target. Because of this war, all citizens of the city are stuck on it as a defence as not to infiltrate the city. Water is gotten from clouds and food is grown on it. Most of the sewage goes into fertilizer while the rest is drained out of a pipe on the bottom of the city. $\endgroup$ – Unhappymarshmellow Oct 29 '17 at 23:50
  • $\begingroup$ Oops I mean that the rope is connected to a sort of trip wire that kills the target once broken $\endgroup$ – Unhappymarshmellow Oct 29 '17 at 23:57
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    $\begingroup$ You are talking about square miles of city. That is going to be some rope. If you float the city by magic, you can anchor it by magic too. If it floats on hydrogen, then you use rope. But then this is 'hard sf' Even your fantasy has rivets. One person defined the difference between science fiction and fantasy: SF has plausible economies. You have to think about how the common people carry out their lives. You can spend hours makeing models and scenarios for ONE comment by one minor character. "I cadged a ride with a load of parsnips on an air freighter from Jersey." $\endgroup$ – Sherwood Botsford Oct 30 '17 at 2:52
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    $\begingroup$ If we are talking fantasy, then the prevailing winds are such that it is above a vortex or perpetual storm (like a tornado or hurricane - think the red spot on Jupiter) that keeps it in place. Or, it is highly metallic, and it is over a particularly strong magnetic field. Or it is magnetic and it is caught up in a conflux of magnetic fields around the planet. Or the conflux of dominant air currents, (wind shear). On earth the jet stream is always reliable and equatorial currents keep the northern hemisphere air separate from the southern hemisphere. The tornado idea also takes care of waste. $\endgroup$ – Justin Thyme Oct 30 '17 at 2:54
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    $\begingroup$ While this "answer" is informative, it does not answer the OP's question. $\endgroup$ – JBH Oct 30 '17 at 5:18
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How would the city defend against a siege, considering the attacker’s intentions are to board and retrieve a person from the palace?

In short, we have ships attacking a city that can't be defended by walls.

First thing we need: look out towers, with a few guards 24/7. They can ring bells to alert from an airbone attack.

Weapons of defense:

First line: Catapults with incendiary ammunition. Straw bales embedded in oil. Iron balls heated in an oven before throwing them. Anything that burns through a long time. If aiming for the hydrogen tanks in the attacking vessels is possible, they will burn. These catapults are only in the outer rim of the city because if your city is made of wood and has also hydrogen tanks, you don't want a burning ship to crash against it.

Second line: Bows and crossbows (the same weapons used in ship to ship combat in the 13th century). Pelt the enemy ships with as many arrows as possible, killing people on board. If the people need to land through rope ladders, they are sitting ducks for your archers.

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Depth Charges: The enemy will attack from above, so send out depth charges attached to balloons.

Suicide Gliders/Fighter: If you have spotted their path of ascent small strike craft designed only to glide or descend might be useful. This floating cities larger size means they could store a massive number of these compared to the attacking ships. Suicide launches would be the most effective, however if your culture is against that they could tow an explosive package behind them, and then either be recovered by allies air ships or glide to staging spots on the ground. (The cost of life might be quite high for this maneuver, but I think resource wise it would be quite effective to swarm the incoming fleet)

Depth Charge Glider: The buoyancy of the depth charge attack keeps the glider aloft, they control the depth charge, aim it, and release them in sequence. Then descend back to the city.

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Giant spinning blades.

This answer is inspired by Game of Thrones and that giant swinging blade they had on the Wall, in case guys tried to climb up the walls. What is the chance something like that would work the way it was supposed to the first time you used it? Pretty silly, but still: pretty awesome.

Around the city is a hoop of metal. It gleams in the sun. It is mounted on an axis through the base of the city. It is sharp; the kids cannot walk out there and sit on it when they are tired of grownups.

city with spinning hoop

When things get hairy, the keepers of the floating city turn it on. It spins up to speed like a blender. Anything approaching gets hit, hard. Grapnel ropes have no hope. Bombs will be forcefully whacked out of the way. Ships will have pieces cut off of them.
The defenders of the city have guns timed to shoot when the hoop is not in the way, sort of like the WW1 biplanes could shoot between the propeller blades as they passed.

Be careful you don't forget about the hoop and build too high. I recommend against going out on the roof of that green building when the hoop is on.

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Here would be my Strategies dependent upon conditions

As with any war, the most effective decisions and strats take into account immediate and developing conditions. But here are some defensive options:

CLIMB!

Take your city to 29,000 ft (the height of Everest). Develop your city with pressure chambers (places where your people could take refuge during sieges that can maintain 1 atm).

At this altitude the human body is dying from cold and asphyxiation. Good luck invading a city when you cant breath and defenders can pop out (from siege rooms) fully rested.

Note: Altitude sickness would likely plague both sides however the defenders having both lived in an aerial city (which would acclimate them) and access to siege chambers would leave them infinitely more able bodied than their attackers. (in other words they would invite them in at this point and decimate them) A single defender could easily take 10-20 attackers if not more under these conditions. And if defenders alternate combat shifts the could easily quadruple that ratio.

Also at 29K ft the air is below freezing, another defensive option is to spray water at attacking ships. This would obviously freeze causing lots of additional weight on the ships. Enough ice weight could be packed on to cause the air ship to go plummeting to the ground. This happens to fishing vessels in the bearing straight causing them to sink.

Note: The Sherpa people have evolved a natural resilience to altitude sickness because of the altitude of their settlement.

Descend!

I don't know presume to know the normal altitude of your city but I assume they avoid violent storms by ascending. In this case if there is a storm present they should descend into it. A big massive city would be hard for a storm to push around however light airships buzzing around it would have difficulty maneuvering let alone avoiding crashing into the concrete siding. These conditions would certainly reduce enemy numbers.

Note: Of course all of this is in addition to obvious conventional countermeasures like archers and ballista.

Finally, I want to re-iterate my opening point. A truly successful defense comes down to the commanders ability to act and react to the conditions of battle. Simple defensive strategies are never enough and this can be best described by summarizing Patton : ' Man has torn down mountains and tamed oceans, of course he can overcome a fixed position'.

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