Soulflame's question thread
https://forums.totalwar.org/vb/showt...t=68970&page=2

3. You can't station artillery on the walls, but the towers are upgradable. One very nice thing is that towers don't shoot unless there is actually a unit on the walls next to it. Yes, this means that if there is only 1 unit garrissoned in a castle, most of the defenses don't work.