This level occupies the map slot MAP01. For other maps which occupy this slot, see Category:MAP01.
From the map start enter either door and climb the stairs. At the top, turn around: You should notice a small rectangular recess in the wall to the top left of the green torch. Shoot at it to remove the bar blocking the berserk pack you saw (behind a demon on high skill levels) at the map start (sector 1).
On the upper walkway on the eastern side of the red door go north to the corner where the two [chaingunner]]s are. Run to the east on this walkway, ignoring the entry to the rocket launcher room, to open a secret switch(A) for a moment. It is located behind the next corner to the left, just in the west of a bloody dead marine. Quickly push this switch(A) to open another switch(B) on the south side of this corner. Again you have to hurry pressing it, just to open yet another switch(C) right next to it. Using this third switch(C) finally lowers the wall (sector 233) that blocked the berserk pack and the blue armor in the southeast of the "swamp yard". Even better, also the wall at the end of the walkway in the north of the dead marine is lowered so you can access the swamp yard without collecting any key.