Final Doom/MAP12: Crater

Blue key
Head through the first door, and cross the room. The blue key is in the first room to your left, but collecting the key will release a chaingunner or mancubus.

Red key
The blue key door can be found if you head directly easy from the blue key room. It will be found on the right-hand side of the room. If you want to collect the box of shells before the door, walk behind the crates and climb up.

The rocket launcher should be in plain view from the blue door.

To reach the red key, continue from the rocket launcher to head outside. There should be about three paths leading north; take the left most one, and take a left turn on the stairs to reach the red key.

The exit
The route to the exit is located past the door east of the rocket launcher. Open this door with the red key, and head down the lift to the maze.

The quickest route through the maze is a simple left turn, with a blue-and-gray wall. Press open to lower the wall, revealing an arch vile.

The exit switch is at the opposite of this final room, behind the monsters and two arachnotrons.

Secrets
There are two secrets, both in the same area:
 * 1) Near the end of the map, you lower a lift containing an archvile.  Go up the lift and head to the south to collect green armor and some shotgun shells.  You can proceed further south for a box of rockets and an alternate route to the start of the map. In multiplayer, there is a plasma gun here.
 * 2) From the same lift, head north for a berserk pack.  Press the switch here to lower the lift.

While not secret, the following places are of interest:
 * In the starting area, there is a brown-coloring on the gray wall. Open it to reveal a Super Shotgun.  The super shotgun is on a lift that lowers to a watery tunnel containing monsters.
 * In the blue room south of the entrance, crossing to the southern end of the room lowers a platform with one or two Barons of Hell. Collecting the supercharge on this lowered platform releases additional monsters.
 * In the blue key room, demons are not released if you do not attempt to collect the chainsaw or stimpacks.
 * A berserk pack is found in the room directly south of the blue key.