How to acquire the yellow key in Doom map Unholy Cathedral:

There are 4 large teleporter pads around the perimeter of
the wide courtyard that dominates the center of the map.
Go to that courtyard, and step on the northwest teleporter
(check your Automap to find the correct teleporter).

You'll re-appear at the northeast corner of a small, square
area, and you'll be facing 2 doors at the opposite corner.
Approach the doors and open the right-hand (west) door, and
step through.  Turn right, to face north; then follow the
left-hand wall along the curving hallway (north, west, and
south), until you come to another pair of doors.  Open the
right-hand (west) door and step through.

Follow the right-hand wall forward (west), until you see a
gap in the floor where some green slime is visible.
Carefully step across the gap, and a part of the west wall
will open, revealing a slime-floored room beyond.  Go into
this room and follow either the left or right wall around
to a teleporter located at the far/north end of the room.

You'll materialize on a teleport receiver pad at the center
of the large courtyard, and you'll be facing northeast.
This teleport receiver is surrounded by 4 large box-like
structures.  The southern-most box should now be open, and
the yellow key can be found inside, near the back wall.

PC Doom players can click here to access a demo that shows
how to access both keys on this level.