I got another one. It might be a little hard to explain:

Everybody knows that if you occupy city towers on a stone wall, they can help you shoot down the garrisons marching on the street. What is less obvious is that sometimes arrow towers can also help you turn the tide on the wall, when you have crap infantry up there.

There are certain kinds of towers that have arrow holes "inward", and there are turns of the wall that those towers can actually shoot soldiers on the wall. You usually notice your men suffering from arrow fires when you rush them to occupy those towers. Exactly those spots.

When you see powerful units coming at you that normally you will die miserably, you can first occupy that critical tower, and park your men to the right spot, where the tower you just occupied can help you shoot down tons of garrison crowded before your men. You might need to plan this during the deployment stage, so you know where you will send your men up to grab that tower and defend in front of it.