Doom II: Hell on Earth/MAP02: Underhalls

Walkthrough
You start at the north of the map, overlooking a water loop. Run across to the other side of the room, killing the few soldiers present, and go through the door. Head to the left and flip the switch to unlock the first exit on the east.

Head down the east side through the water loop. There is an alcove on the right-side with a few Sergents, and if desired, you can head to the path on the left side to a room with many soldiers and a Super Shotgun.

At the end of the loop is red key shown inside the building, heavily guarded by soldiers. To enter the building, head to the left, and run into the side entrance. From here, you can collect the red key. Note that entering the side window opens a closet within the water loop, and collecting the key opens two nearby closets outside the building.

Return to the north control room, and open the red key gate. Flip the switch to open the west half of the level. Exit the north room to the west, and head to the south end of the loop. From here, you can drop down the north-west or south-west hole. Both exits have imps watching them. The key is located in the south-west hole, and once you get the key, you can head to the north west to the blue key gate.

The exit should be just to the left, although it is guarded by Sergeants.

Sectets
Only one secret in this map. In the red key area, search the wet wall at the top of the stairs, to reveal an area with blue armor. This door is only visible on the map. (Sector 79).

Multiplayer

 * A rocket launcher is near the blue key.
 * A plasma gun is near the red key.
 * A shotgun and soulsphere is in the control room just off the northern area.
 * A chaingun is in the first eastern alcove in the water loop.