Context : my fictional universe is set in a time period akin to the european Middle Ages/Renaissance. Think of the 1400s-1500s for reference.
In most aspects (political, social, industrial etc.) my fictional universe closely resembles real life 15th century Europe, with two important clarifications:
- Contemporary technology and metallurgy has allowed a given kingdom to develop steam-powered machines. All other innovations that can derive from this aside, steam power has in particular led to the development of steam-powered airships, that is vehicles closely resembling merchant ships or carracks but that are airborne using airbags/balloons. These airships are also used for war, i. e. for reconnaissance, transport, or bombarding targets.
EDIT: To clarify, the steam and airship technology is enough for the airships to be effectively airborne (in a fantasy setting), but still relatively primitive enough for the airship's to be slow, clumsy, and with a low flight ceiling (let's say about 200 meters high). Also, their carrying capacity isn't too great and they can only carry about 10 to 20 men or the equivalent of a small to middle-sized fishing boat in terms of merchandise. However, they can be produced in relatively high numbers and are not a rare sight on the battlefield. Also, rival nations have started developing airships on their own as well.
- Gun powder does not exist in my fictional world. Whether this is because it hasn't been invented yet or because the universe's rules do not allow for this specific chemical reaction, Humanity simply does not possess it. So the main projectile weapons in use are the traditional ballistic weapons such as ballistas, catapults, mangonels, trebuchets, (cross-)bows, etc.
Assuming whichever kingdom that invented airships first gained a decisive tactical advantage thanks to them, and used them successfully against traditionally-built European medieval castles (for example by bombarding the castle structure and defenders with stones/arrows/other, lighting the castle on fire from above, or airlift and then drop invaders within the castle walls), how would the architecture of castles evolve to adapt to the threat of an attack from the skies ?
If you wish to contribute, feel free to give suggestions for active defense (i.e. weapons to fight the airships) as well as passive, more structural defense (i.e. the structure of the castle itself). I also gladly take suggestions/examples from other styles of castle-building (Japanese, Middle-Eastern etc).
Edit 2.0 : Thanks a lot for all answers ! I realise my premise is not the most plausible and may thus appear convoluted and difficult to work with. Nevertheless, I really appreciate the answers that tried to make do with my set premise as well as the various reality checks other contributors have given (which may help me make my setting more plausible/realistic). Cheers !