To get the yellow key on level 2, THE TERRAFORMER:

Once you have the red key, activate the Terraformer (the
cluster of 4 large pile-drivers).  The Terraformer creates
a large pit in the ground beneath it.  Jump down into the
pit, and enter the subterranean hallway you'll find there.
Go to the far end of the hallway, and you'll see a switch.
Activate the switch to open a passage on your left.  Go
into the passage and up the stairs.  At the top you'll find
a red-keyed door; Open it and enter the small courtyard.
At one end of this oblong courtyard you'll see some windows
that overlook the courtyard.  Below these windows and to
the left is an opening leading inside.  Step inside, and go
up either one of the curving stairways.
At the top of the stairs is the entrance to one corner of a
large squarish room.  On the far side of the room, opposite
the stairs, you'll see yellow key up in a hole in the wall.
Walk up to the key and turn right.  Ahead you'll see a box
against the same wall, with 4 small red lights above it.
On the wall opposite that box (back on the other side of
the room) is a switch.  Activate the switch to lower that
box and reveal a teleporter.
Go step on the teleporter, and you'll reappear in the room
that has the windows that look down into the courtyard.
In this room look for a switch on the wall and Activate it.
This causes the yellow key to drop down to floor level back
in the room you were in a moment ago.
Step on the teleporter to go back to the room, and grab the
yellow key.

Hope this helps. :)                               

Support for console and PC Classic Doom games
Back to Doom Help page