If you run through the towers and they're is no enemies laying claim to any of the surrounding area i.e. there are no enemies around, you should be able to capture them, and the arrows will start shooting down on the enemy.

I usually send a unit of skirmishers up on my towers and have them run around the walls anywhere I think I'll have to walk my troops. You can also capture the gatehouse, so hot oil won't drop on you. This is usually more difficult though, as there are often troops stationed at the gatehouse you start near, and walking all the way around would make sieges even more boring than they already are. Be careful though, because if your troops start fighting near the gates and you own them, hot oil will be spilt on enemy troops and your troops might get burned as well. Hope that helps.