Do you have a large skybox around the map or something? The map itself doesn't look too complicated, but from the top-down view, you shouldn't be able to see the top/outside edges of the walls like you can. Make sure you seal each room as best you can. put sky brushes down to the walls so they block areas off from each other.
Also, as KnacK hinted, having a single path from one base to the other will lead to poor gameplay.