naive sable
Fortresses are kind of meh for the time being, they're just larger camps which are not good enough to be actual cities. They could be proper defensive places instead with a few tricks.
My idea for fortresses is to turn them into support structures for your armies, easily defensible but faster to assault than cities.
||So if it takes 10 turns to assault a city, you could parapet yourself in a castle and last 5 at most, knowing you'd have the odds in-battle slightly in your favour but you'd lose the more valuable city if you turtle in it forever. ||
Here are some building ideas:
Tower of Sorcery: Increases the range of world spells by 1 province when cast from this castle. (this could work for Wizard Tower too)
Banquet Hall: Allows troops to regenerate some extra hp per turn while in the castle.
Barracks: Allows troops who force-march into fortresses to not suffer from penalties other than the loss of hp. Troops who force-march from this castle don't suffer any penalties either (only the army inside the castle).
Training grounds: Allows any troops in this castle to passively earn exp. Not much, but enough.
Guard post: Allows the training of tier 1 troops in a castle. Can be upgraded for tier 2 troops at most.
Keep: Adds a keep (decoration) in a small mound with walls around it. The door can be attacked, but only strong creatures can damage the walls. Ideal for last stands.