How to reach the exit in the Master Levels map GERYON:

You begin this level in a dark room at the south edge of
the map.  Leave this room through the doorway in the west
wall, and you'll drop down to the ground below.
Turn right and go through the door you'll see there, and
enter the long rectangular room ahead.
Go forward (north) and up the curving stairway at the far
end of the room.
At the top of the stairs you'll emerge at the west end of a
large, oblong outdoor area.  Go east, to the far end, where
you'll find a door.
Go through the door to enter the south end of a rectangular
room.
Go to the north end of the room and look in the northwest
corner for a curving flight of stairs.
Go down the stairs and through the door at the bottom.
You'll enter the south end of another room.  At the north
end is a locked door, and on the east and west walls are
large, square panels.
Activate the left-hand (west) panel like a switch, and it
will open.  Step through to enter another room.
There are open doorways in both the northwest and southwest
corners of this room.  Go to the left-hand (southwest)
doorway and step down through it, to land in a corrosive
stream.  Immediately Run along the stream to a platform
you'll find nearby.
Activate the switch you'll see here, and then Run back the
way you came along the stream, to the doorway you just
stepped through a moment ago.
Activate the switch here to lower a small lift that will
let you get back up and through the doorway.
Turn right and go to the wall panel you came through a few
moments ago.  Go through, and you'll find that the other
wall panel, on the east side of the room, is now open: Step
through that panel to get the blue key.
Once you have the blue key, go to the locked door on the
north side of the room, and go through.
On the north wall of this room are 2 switches.  Between the
switches is a semi-camouflaged panel.  Open this panel and
step north into the next room.
Go to the middle of the room and you'll see a stairway to
the north.  The east and west walls of this room will also
open, but these wall panels are triggered by switches that
are located in the next room to the north.  (Remember the
location of these panels; in a moment you will need to Run
through the west one.)
Go north down the stairs into the next room.
Turn left (west), and Activate the switch you'll see in an
alcove ahead of you.
Next to you a lift drops down momentarily: Get on, ride up.
At the top, look southeast for a nearby switch.
Activate the switch and immediately step left (northeast)
out the window, and Run south, up the stairs, and turn
right to go through the open panel on the west wall of the
room.
Follow the short path to the teleporter and step on.
You'll appear in a circular chamber at the northeast corner
of the map, and you'll be facing north, towards a switch.
Activate the switch, and a wide circular column behind you
will sink down, revealing a narrower 4-sided pillar in the
center of the room.
On the south face of the pillar is a switch.  Activate it,
then go to the teleporter at the south side of the room,
and step on.
You'll appear at the south end of a large open area at the
northwest corner of the map.
Go down the steps ahead of you, then go up either one of
the curving stairways to the east or west.
Follow the stairs up to the platform to the north.  At the
middle of the platform, turn south and go down the narrow
walkway to the switch ahead of you.
Activate the switch to open a square chamber behind you to
the north.
Go north to the newly opened area.  Step on the teleporter
there to exit this level.

Done!

PC Doom players can download a demo of this level, which
shows how to access the exit, by clicking here.