This is some kind of jail. You wonder why the hell are monsters inside the cells, after all, they are the bad guys and you are the good guy(s). Your serial number is: 2.718281828.
From the square courtyard early in the level, go through the door to the north. Follow the corridor and take the first right. There's a cage in the wall to the right with some monsters in it. Open a section of the wall opposite this cage to reveal a secret passage with a backpack.
Open the back, "dead end" wall inside secret #1 to gain access to another caged room, where you can pick up the chainguns belong to the commandos inside.
South of the crusher with the chainsaw on it is a water-floored corridor in the shape of an inverted "T". There is an alcove in the eastern arm of the T-shaped corridor with a sergeant in it. Open the eastern wall of the alcove to get a computer map.