Arrows killing occupants?
After all, it makes sense that a few arrows/bolts are able to break through a weak spot in the walls or through the windows and kill whoever is inside...
This would make towers less effective while still being useful (sure, you no longer need to completely destroy them, but you still need to focus them first while the enemy hits your troops).
Also, giving Crossbowmen's bolts an higher chance to break through would differentiate them from Bowmen, and give a reason to rationally choose if you want more Bowmen or Crossbowmen (i.e. Bowmen would be better in an open field battle, while Crossbowmen would be better in taking care of the recruits inside the Towers, thus being better during sieges).
Any opinion?