Walkthrough for the "Tribes of the East" expansion.

Campaign 4, mission 2 – I can't figure out how to complete it after 3 days. (I move the city, defeat everyone, and go into the dungeon, but I keep hitting a dead end. I've found all the obelisks. The map has some kind of structure; I run there, and there's a crystal and a head, and in the middle, something that looks like a crystal too, but I can't get it. What should I do? Please provide more details. Thank you in advance.