this was easy, you do not have to go to a bunch of walls. I just picked one, get it to 79% health and then rightclick your mortar clicky. When your coming down the spiral walkway there are paths off to the side as your coming down. Go down the path and the wall is there in front of one of the doorways in my case. There is a mob that roams past but even though it aggros, it doesn't hit or stay on you it continues to roam so a merk works find for this part of the quest.