A tower you build that has a radius circle like a beehive.
It will have permissions like claims.
Anyone not on the permissions will be shot when in the radius.
You will have to fuel it by putting arrows, or maybe some sort of stone thing into the towers.
Walls right now are just obstacles and not a real defense. This would actually defend against attackers, because they'd be getting pegged by arrows as they sat there trying to destroy the wall.
EDIT: I just realized people would march a bunch of noobs at walls to make towers waste ammo. So instead of firing at anyone not on the permissions, it will fire at anyone not on the permissions committing theft or vandalism crimes within its radius.