Access the keys and the exit in Master Levels map Virgil:

You start in a square chamber near the middle of the south
side of the Automap, facing north.
Walk forward a bit and notice that to your left and right
are two demon-faced walls.  You can walk up to either face
and Activate it like a switch.  When you do this, the wall
in front of you drops down.  Behind the west-side demon
wall is a chamber; behind the east-side demon wall is a
door.

Go through this door, up the steps, then turn left and head
north to a large demon-head/pentagram door.  Go through
this into a courtyard.  The courtyard has several doors; go
through the north one, up the steps, and look for a door to
your left (west).  Go through that, and follow the corridor
around to your left.  You'll come up against a locked door
that you need a blue key to get through.  Look to the left
of this door for a hidden panel in the east-side wall of
the corridor.  When it opens, Activate the switch you'll
see behind it.  This will cause changes back in the room
you just left, and reveal the yellow key.  Go back into the
room and grab the key (it'll be on the floor in the
corner), then go through the east-side door, into a passage
that heads east and then south.
Open the door at the south end of this corridor and walk
into the circular chamber ahead of you, and look for a
switch on the east wall.

Activate the switch.  Now go out through the west-side door
of this chamber, which leads back into the courtyard you
were in awhile ago.  Look for a narrow open door ahead of
you and a bit to the left (it's in the southwest corner of
the courtyard).  There is a teleporter here; step on.
You'll be teleported to a square chamber in the southeast
corner of the map.  The blue key is here; grab it.
Again, you'll find a pair of demon-faced walls on opposite
sides of the room.  Activate the west-side wall and it will
drop down to reveal a door.  Go through and up the steps,
turn right, and head out through the door on the north
side.
You'll be back in the courtyard you were in awhile ago.
Turn left and go through the big door in the middle of the
west side of the courtyard.

Ahead of you will be a circular chamber with a cage in the
middle.
On the north side of the cage is a barrier that you can
activate like a switch.  It drops, allowing you access to
the cage.
There are four slab-like structures inside the cage, which
run floor to ceiling.  These slabs surround the red key,
which lies on the floor in the center of the cage.
On each slab is a face-like switch.  Step up and activate
the switch on the slab in front of you.  All four slabs
will drop down, and allow you access to the red key.  Grab
it, and let the slabs rise up again around you.
When the slabs are back in position, surrounding you, turn
and activate the switch you'll see on the slab to the west.
This will open a west-side door in the circular chamber
surrounding the cage.
Activate the switch on any other slab to cause the slabs to
drop down again.  Step out of the cage and go leave the
circular chamber through the now-open west-side door.
You'll be in a large, rectangular courtyard which fills
most of the west side of the Automap.

In the northwest and southwest corners of this courtyard
are a pair of elevators.  Get on either one and ride it to
the top.  Now walk along the wide ledge at the top, and
look for a door on the west wall.  Go through and you'll be
in a small, irregular chamber.  When you walk around on the
floor of this chamber, you'll cause a switch to rise up out
of the floor, near the chamber's west wall.
Activate this switch to open the path to the exit.  (The
exit is in the form of a large teleporter pad on top of a
big square pedestal out in the open area you just left.
The teleporter is blocked by bars on the west side of the
pedestal.  When you activate the switch, the bars drop.)
Turn away from the switch and Run east (remember your Run
button), through the door of the chamber, off the ledge,
and on to the top of the pedestal.
With a little luck, you'll land on the exit teleporter.  If
you miss your jump, just go back up to the switch in the
chamber and try again.

PC Doom II players can access a demo of this level showing
how to access the keys and the exit by clicking here.