Nariko must prevent Bohan's Army from taking down the walls. He will send catapults at you along with flaming barrels. Use your cannon to take out all of them.
You will see the wall's status in the upper right hand corner of the screen.
The barrels explode if you put a cannonball anywhere near them. The catapults are a bit tougher than the ones you fought before.
Hit the two shields to take out the catapults wings. Once those are gone, concentrate your fire on the big metal grill at the front of the catapult.
When you've hit it three times it will blow up and then you will gain access to two internal shields inside of the catapult.
There's a lot to do here, and you have to make every shot count. Take out the ones in front as quickly as possible.
The more distance you get between them and yourself, the easier it is to afterTouch the cannonballs to them.