Necropolis – “Spider’s Trick” [Campaign 2 – Mission 4]

I can't complete the first quest in mission 4, where I need to capture cities (6 or 7). I've captured all of them except for the first one on the list; I can't find it, and I've already searched everywhere...
If anyone has completed it, please help. What is the entrance in the center of the map? Could that be the reason...?