Destroy the first siege tower with the catapult near the middle of the wall. It requires two hits to topple. For the second tower that rolls over to the far right, you can just use your projectiles to take it out. Keep firing at it in order to bring it
down.

When you have destroyed the third siege tower, go to the fourth one and kill all the Orcs that are in there until you cannot see any more. Then, shoot down the siege tower with your arrows. Once that is done, go to the men that are guarding the stairs. Walk down there to complete the level.