Doom/E3M4: House of Pain

At the start, there is a rocket launcher and three paths in different directions. The right branch leads to a shotgun behind a door. The left path leads to a berserk pack and blue armor.

Continue up the steps in the center path to reach the central room. There are useful items within the pits, which may be collected by flipping the two left switches (as long as you do not trigger the crushing ceiling). To open the southern region, flip the right-most switch, and continue.

The southern section branches into three corridors. If desired, you can head into the northern corridor and open the right-hand door for items. Unless trying to kill all monsters, avoid the left door as you will need to fight a cacodemon to exit.

Collect the blue skull key along the southern corridor, then head to the central one to open the door. From here, the path branches into two directions. The north leads to monsters, a few powerups (including invisibility and supercharge) and two secrets described below.

When you continue to the right, you will arrive at an apparent dead end. Walk along both edges of the blue pool in the bottom right, then return to the front of the door. Run across the red floor under the double doors. While quick players can run directly from the side or blue pool, the second door will close if you try crossing the red area from the side.

Flip the two south-west switches on the left, and get the yellow skull key.

Flip the two south-wet switched on the right red pillar, and head to the red skull key.

Proceed down corridor until you reach the end. Drop down on the right-hand side, and run to the end of the corridor to reach the exit.

Secrets

 * 1) In the left path from the start, the teleporter allows you to attack a few monsters from behind.  Aside from the surprise attack, there is nothing special here.
 * 2) After the blue key door, there is an optional branch to the left.  Continue along, and you should reach a large room; behind the right door is a small lava pit and a lowered BFG.
 * 3) In the left path of the same room is a few barrels.  Open the angled wall at the end to reveal some rocket boxes, and a few demons.
 * 4) After the red key gate are three alcoves on the southern wall.  In the third-most alcove is a radiation suit, which helps running down the corridor to the north.