How to get to the exit on level 20, Gotcha!:

Note: There are no keys anywhere on this level.

The first step in accessing the exit is getting onto the
high, curving staircase rising out of the red acid in the
northeast corner of the map.  This can be done two ways:

Method A: Use the teleporter located in the middle of the
north half of the map.  It's the large demon-faced teleport
pad on the low circular walkway outside in the acid.  Step
on this pad to appear at the top of the stairs... Or:

Method B: Look for a red teleport pad hidden in a niche in
the wall just a couple of paces to the south of the curved
stairway, down in the acid.  This will beam you to a small
room just above the low end of the stairway.  Carefully
jump from here down to the stairs.  As you land, don't let
momentum carry you off the edge.

Once on the stairs, turn to face east, so that the black,
square opening of a cave is directly ahead.  Carefully back
up a couple of paces to get a running start, then Run to
jump into that black square hole ahead.  (Remember that Run
in PC Doom games is typically enabled by holding down the
Shift key.)
Once you're inside, follow the right-hand wall around to
the right (south), and the wall ahead of you will rise up,
revealing a teleport pad.  Step on.
You'll appear at the top of a tall, narrow elevator, which
will immediately begin to drop down.  It will lower you
into a small oval-shaped room where one or more Sergeants
will likely be waiting.
After the elevator drops, go to the wide window in the east
wall, and walk onto the small "balcony" there.
When you step through the window, you'll see that a narrow
vertical section of the building ahead of you (to the east)
will begin to drop down.  When that section lowers all the
way, a new opening/doorway is uncovered at ground level.
Run east off the balcony towards the building, and go into
that new doorway.
You'll be entering the northwest end of a short hallway.
Walk to the opposite end, and that doorway you just came
through will be automatically closed off.
At the end of the hall, turn left to walk up the stairs.
When you get to the top of the stairs, a teleport booth
will open, back at the northwest end of that short hallway
you just walked down a moment ago.
Go to that teleporter and step on.  You'll be beamed to a
bullet-shaped building on the east side of the map.  You'll
be facing north.
Ahead of you, up some stairs and across a zig-zag catwalk,
is a small chamber with a large open hole in the floor.
Step into this hole to exit the level.

To read about or download a demo of this level, showing how
to exit, click here.