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