This level occupies the map slot MAP05. For other maps which occupy this slot, see Category:MAP05.
In the area behind the first door, there are 16 skull switches on the walls. Push the wall below the second skull switch (the one with the non-lit eyes) from the south on the western wall to open an alcove (sector 47) below the southernmost skull switch on the same wall. Inside is a super shotgun and four sets of shotgun shells.
In the area where the blue skull key is found, there are four pairs of skull switches on the southern wall. Shoot the easternmost one (again the one with non-lit eyes) to open another alcove (sector 201) to the left of it. It contains a soul sphere.