Proceed through the first room and bear left along the corridor until you reach a slime pit. Jump in.
the slime pit has one exit: this leads into a sunken slime area with a walkway on the right extending around to the left. At the end of the slime pit is a lift. Use this to get up onto the walkway.
The walkway leads through to a series of four rooms of the same dimensions as the first. One of these contains a switch: this opens the door in the other.
Behind the door is a walkway over another sunken slime area, flanked by two other walkways. Each walkway can be accessed by lifts to the rear. Depending on the skill level, a red key will be situated on the left or right walkway.
Once you have obtained the red key, return along the first walkway to the red key door.
Behind this is a teleport back to the start of the level. Proceed as before, but turn right before the slime pit.
The exit is behind this door.
There is a Super Shotgun hidden behind a fake wall in the start room.