@RogerCooper & @Cernel: You are both right about castles. The first idea that comes to my mind is a two-hit static Castle unit which, when destroyed, changes into a three-hit Ruined castle capturable infrastructure, setting transfer attributes to true. Whenever this unit takes 2 damage (and would normally be destroyed) it changes into a 3-hit unit, which still only has 2 damage, so it will survive. Then the winner captures it, and next turn it gets repaired back into a non-ruined Castle. Essentially this castle would be a capturable infrastructure that can also defend itself to an extent.
I had another idea about medieval warfare which I wanted to experiment with, but that is unlikely to happen now. In short, you can think of levies or garrison as units with limited effective range. So you just make them non-kamikaze air units with 1 movement. Now they can move around but can't attack. But if castles give them 1 bonus movement, then suddenly they can launch attacks from castles against directly adjacent territories. It would be rather pointless for the player to move them away from castles (maybe give them 0 defense without a support attachment from a castle, to emphasize this). This would represent castles as dynamic defense systems, which is closer to historical reality than a static heap of bricks.