Maybe being underground somewhat as well as being heavily destroyed the ship is in two pieces. I mean for the most part it would seem you have an entire engine section split into 3 different locations; the engine room, drive cylinder, central core. These to me seem all to be just engine components. So maybe after you get through that you go into a large area where your outside the ship making your way to the Main Bridge across some subterranean space as well as the remainder of the ship.