To start this level, shoot the marines and Imps through the grates to either side of the corridor ahead of you. Go along the corridor and you'll be in a small outside area. You can't go left, as there are a pair of red keycard doors at the end. Head right, and kill the various enemies in the long room. At the end, grab the red keycard, and immediately turn around and pull out the chaingun - there's a lot of ammo for it in this room, and you'll need it for dealing with the hordes of assorted enemies that come piling out of secret areas in the walls. Once they're all dead, pick up the remaining goodies and head straight back across the open area to the red keycard doors, and open either one of them. Kill the enemies in this corridor section, and at the end watch out for the small room set back into the wall opposite the blue keycard as it contains two Imps and a Demon. Grab the blue keycard.
Looking out across the slime pit, drop down off the end of the platform where the blue keycard was and hug the wall to the left around until you get to a secret area with combat armour. You can head through the corridor down here for some more goodies, ending with a Light Amplification Visor, a Partial Invisibility Sphere, and two boxes of rockets. However, do not use the switch to re-emerge near the start of the level. Turn around and go back to the edge of the slime pit. Dash across the slime pit around to the other side of the room (around the right-hand wall, past the blue keycard platform) to pick up a Radiation Suit, then head through the low tunnel opposite where the blue keycard was and go up the lift at the end for a Soulsphere. Go back to the corridor in the left-hand secret section, follow it to the end, and use the secret door to emerge back at the start of the level.
You will now want to go straight on and through the door. Clear out the room of assorted enemies. The door at the far end past the computer pillar is a yellow keycard one, as are the two in either corner of this room, so don't bother going that way. Instead, leave the room to the right (looking from where you entered). In this outside section, kill the marines on the high platform and snipe a few of the Imps beyond the large grate on the other side of the area. Head across the walkway and approach the blue door. Turn around when you get near, as the high platform will have dropped revealing more enemies and a secret room containing another Radiation Suit.
Go through the blue door once you've dealt with these enemies. You'll be in a series of cross-shaped walkway slime rooms and dark tunnels forming another maze. Find your way to the opposite side where you'll see an illuminated switch - this opens the maze's exit. You can also take a hint from the arrow formed by a group of cleverly-placed armour pickups, and open a secret wall for a Computer Map. Once you've cleared the place out and thrown the switch, you can leave the maze to the room beyond the large grate you saw from outside, and pick up the yellow keycard in here. Once you do, turn around quickly and kill off all the enemies that come out of the wall section.
Find your way back through the maze to the outside again. A load of Imps will come out of a previous dead-end in the slime - kill them, then follow the slime path all the way to the end for some goodies. You can then go through the door, kill the shotgun marines, and drop down. You'll be outside the section of the level originally blocked off by the yellow keycard doors, and you can get in there and obtain a Soulsphere whilst you're at it by lowering the pillar the Soulsphere is on and then opening the wall. Be careful however, as there are a lot of enemies in here. Clear out the place, then go right from where you entered the area and throw the lit switch to open the next area.
Back in the main area of this wing of the level, a new door has opened which some enemies will appear from. Kill them, then go down either of the side tunnels to the room at the end. In here, kill off the enemies, then approach the switch in here. A load of doors will open in the walls, revealing lots of enemies. Deal with them all, then hit the switch. Go in some of the hidden wall sections until another one opens, revealing a load of Spectres and the exit door. Kill them all off and go through the exit door. Inside, kill the last Spectre and leave the level by throwing the exit switch.
- The blue keycard at the bottom-left corner of the map overlooks a slime pit. Dive in and charge forward - you should get to an area with three armor bonuses and a super charge. Only the armor bonuses count towards the secret, the super charge is optional. (If you don't get there in time, head back to the entrance to the tunnel and go back in.
- The exit to from the slime pit is to the right. Walking to the exit is a secret, and contains a mega armor, and leads to invisibility and light amplification goggles.
- You first need to enter a corridor leading to the blue key door. When you return, a lift to the south lowers, giving access to a secret from a door in the back, with invisibility, a backpack and a rocket launcher. If you have the blue key, it also provides access to the yellow key area.
- After crossing the steps leading to the yellow key area, a section north of the slime pit opens. Enter it to collect ammunition, a backpack, and health (claiming the secret). Behind the one-way door to the north is access to a supercharge.
There are no changes in the item layout for deathmatch. However, the weapons are located:
- Shotgun, near the entrance (ignoring various monsters)
- Chaingun, in the red key area
- Rocket launcher, in the yellow key secret area.