E1M9: Military Base is the secret level of Knee-Deep in the Dead. Designed by John Romero and the par time is 2:45.
From the start point, get rid of the monsters in the middle and make a U-turn left and left again. You will see the yellow key, take either left or right paths to get behind the fence and get the yellow key. Get to the southern part of the level, then go mideast to get to the yellow door. Go through and grab the red key. Go midwest now to find a red door. Go forward then left until you see four barrels. Press the switch (exploding the barrels is optional) then blow up three more barrels to the left. Follow the route back to the room, there will be a lot of demons, use the barrels as a defense, blow them up, then grab the blue key. Now find the blue door at the north. Go through, you will pass the exit switch, inaccessible across nukage between the exit and where you are, fall down into the slime river, follow it to a switch. The switch raises a bridge to the exit door. Quickly kill all the monsters in the opening to the right, if the door closes, step in the switch area again, then follow the opened door to a lift. Lower it, ride it to the top, open the door, and you should see a bridge raised. Get to the other side using it. Press the switch.
Other points of interest:
Once you grab the red key, a trap will open.
The northwest of this level has a star with some health and such on it. Once you step on it, however, an ambush of monsters will teleport.
There are actually two yellow doors and two red doors. You can take either yellow door and either red door. Rather you should take both to see if there are any monsters to kill behind.
To the east of this level is another passage that has a lightened floor. Stepping on it lowers it like a lift. You will see some imps overlooked by a large nukage pool. Once you grab the health to the left, the imp platforms will lower.
- To the southwest of the eastern part of the level, or the northeast of the southern part of the level, are two hidden lifts. Ride either one down to access a secret containing a chaingun.
- To the mideast of the room behind the blue dooor is a different-textured wall. Open it to access a room with some weapons, large ammo pickups, and a backpack. (to get on them a lift will lower and ride it to the top, but to actually get these items you must straferun or run)