Too see how it works, go into a custom battle or wait for one in the campaign and move your men around the fortifications during deployment, you'll see a flag pop out of the tower showing it is activated The tower gates will always be active as long as you have 1 unit within a certain distance of the gatehouse but as the defender you have to control both the top of the gatehouse and behind it, the attacker only has to capture 1 to gain control. Also a routing/fighting to the death unit will cause the owner to lose control of the tower.