MAP03: Main Engineering (Doom 64)

MAP03: Main Engineering is the third level of Doom 64, designed by Danny Lewis. The level is relatively small, consisting of only four small rooms where you need to activate teleporters or press switches to receive keys. It uses the music track "Tortured Transmission". The level also introduces cacodemons, lost souls and a brand new enemy exclusive to Doom 64: the nightmare imp. This level is unique in that two of the rooms' structures end up changing either moderately (the center room with the blue key) or dramatically (the second to last room, where the player finds the switch which gives them access to the red key).

Walkthrough
Kill the zombieman in front of you and press the switch to open the doors at both sides of you. The surrounding floor contains several nightmare imps. Pass the door in the middle and enter the room carefully. Wait until the central platform has risen and the walls to your sides have opened - revealing more normal and nightmare imps. Get the blue key on top of the platform. Some ammo clips and stimpacks are available in the teleporter corners.

Pass the blue door, face several nightmare imps and cacodemons, and activate at least either one of the teleporters in the corners of the central building by pressing switches in its side walls. You will appear on the upper walkway, where you can find stimpacks in the corners. Getting them opens closets at the ends with a stimpack and a box of shotgun shells, or a box of bullets on the other side. A switch in the center appears, with more ammo in front of it. Pressing the switch opens the central room, releasing more cacodemons and lost souls, and revealing the yellow key. A lift is available at the side opposite the entrance, once the room is clear of monsters. You may return to the starting point to find a backpack there.

Return to the platform where the blue key was kept (more normal and nightmare imps await you) and go through the yellow door, taking a chaingun and a box of bullets. Press the switch in front of you, and the ceiling in the room ahead will appear to lower and press a rectangular hole into the floor. Once you can move past the switch, step into the room, killing the imps and lost souls that appear, and pick up the rows of health and armor bonuses.

After finishing them off, descend into the centre of the room. The right switch briefly opens a niche with stimpacks and boxes of rockets opposite to the entrance. Pressing the left switch has no obvious result yet. Return to the previous room and find the red key on the same platform where the blue key was found. Enter the corridor behind the red door to find more nightmare imps and the exit switch.

Official

 * 1) After the platform with the blue key has risen and the staircases are accessible, hug the outer wall of the right staircase to find a secret area with an invisibility sphere behind a fake wall near its middle corner.
 * 2) Do the same for the left staircase to find an invulnerability sphere at its top.
 * 3) In the room behind the blue door, touch the left computer screen in the wall to the left to open a niche to the right of the entrance. A box of shells, a berserk pack and a chainsaw can be picked up inside.
 * 4) Leaving the ammo niche in the second-to-last room also briefly raises a tiny pillar in the right corner from the red door, which allows you to climb the edge in front of the wall that opens to enter a hidden yellow passage visible from the red corridor. It contains a box of bullets, shells and rockets.
 * 5) In the corridor with the exit switch, interact with the second last computer screen before the exit switch to open a hidden niche directly behind you. A medikit and a mega armor are sitting inside.

Non-official
After pressing the switch that alters the yellow room's layout and spawns in several imps and lost souls, walk past the said switch (now lowered) and immediately turn around and exit the yellow door. A soulsphere will appear outside the yellow door on the large central platform where the blue key and rocket launcher previously sitting on. If it is not collected before activating the switch that triggers the appearance of the red key, it will be permanently replaced by the red key.