Level 56: The Military Base

From the start, launch a Rocket into the cage of Imps and barrels to eliminate them.

Follow the left-hand wall to an opening. Kill the Zombiemen and Shotgun Guys, then descend the stairs.

Eliminate the Imps, Shotgun Guys, and Demons in the lower room and retrieve the Yellow Key from the center.

Return up the stairs, kill the remaining Zombiemen and Shotgun Guys, and proceed through the opening to your right.

Head down the stairs. Turn left at the bottom, approach the corner, and the floor will lower.

Collect the Chaingun and health in the middle of the revealed area. Return to the lift and ride it back up.

Go through the opening to your left, then descend the stairs. Open the Yellow Door and proceed through.

Use a Rocket to clear most enemies in this room, then finish the rest with another weapon.

Retrieve the Red Key from the middle of the room. A wall will open, revealing Shotgun Guys and Imps; eliminate them with a Rocket and another weapon.

Collect the Medikit and Stimpacks if needed, then exit the room the way you came.

Back in the previous area, open the Red Door at the far end and go through.

Clear most enemies with a Rocket, then deal with the remaining foes. Proceed through the opening at the far end of the room.

Destroy the barrels to reveal and hit a switch.

Exit this room; the central pillar will have lowered, revealing the Blue Key and Demons. Kill the Demons with a Chaingun and take the Blue Key.

Go through the opening to the left. Return to the level's starting area and proceed through the opening at the opposite end.

Open the Blue Door in front of you and go through.

Across from you, deal with the Imps and Shotgun Guys. For Spectres, use a different weapon. Drop down into the pit below.

Follow the hall. Once you see a platform with an Imp on it, quickly get on the platform and hit the switch.

Turn around; a wall will open behind you. If it closes, return to the platform to reopen it.

Follow the hall, killing Imps, to a lift. Lower the lift, board it, and ride it to the top.

Open the door in front of you and go through. Eliminate the new Spectres.

Cross the new bridge to your right, collect the Medikit, and hit the exit switch.