How to get into the exit on level 11, Circle of Death/'O'
of Destruction!:

First, pick up the the blue key.  It's in a small elevated
room on the far east side of the Automap.

Now get the red key, and then go to the red-keyed door at
the northwest corner of the map.  Below are two common
methods used to accomplish this:

Method 1: After picking up the blue key, go west down the
stairs, and continue west, back across the short bridge to
the raised circular pathway.  Turn right and walk a few
paces north.  Turn to face east, and look for a new open
doorway at the base of the wall in front of you.
Jump down into the slime and Run into the doorway.  Follow
the curving tunnel around to a teleporter pad, and step on.
You'll materialize on a tall, narrow platform near the
center of the map.  You'll be facing southwest, towards a
lower, square-topped platform, where the red keycard sits.
Run off the platform you're standing on, to land on the
other platform, and grab the key.  (Remember that "Run" in
computer-based Doom games is typically enabled by holding
down the Shift key.)
Turn to face west, and look for a tall, four-sided green
pillar ahead of you, which is standing flush against the
interior side of the circular walkway.
There is a switch on the north face of the pillar, near its
base.  Jump down into the slime and Activate this switch.
A section of the circular walkway will drop down next to
you.  Run northwest across the lowered walkway, and across
more slime, to the threshold of the red-keyed door.

Method 2: Look for the blue-trimmed door on the west side
of the Automap, and open it with the blue key.  Go through
and continue into the room off to the left.
Look for a switch on a pillar in the middle of the room,
and Activate it, then walk south and east to the open
doorway ahead.  Carefully step through, and you'll find
yourself on a rising walkway.
Follow the trail of health potions to a switch that you'll
find on the east wall of a semi-enclosed area ahead of you.
Activate the switch to raise a north-south bridge that
leads from the circular walkway to the isolated platform at
the center of the map.
Once you Activate the switch, step back out of the shadowed
enclosure and turn right.  You'll see a stairway running up
the south edge of the raised circular walkway.  Jump down
to the base of the stairs and go up to the walkway.
Follow the walkway around to the bridge that leads south to
the central platform, and get the red key there.
Walk back to the circular pathway, and walk around to the
northwest part.
Jump down and Run to the threshold of the red-keyed door.

Go through the red-keyed door and turn right.  Walk east,
and the thick bars in front of you will retract to let you
pass.  Follow the stairs (on your left) up and around to
where you'll find a switch on your left.
Activate the switch, and a couple of barricades nearby will
drop down.
In the center of the north side of the map is a large
raised walkway (it'll be out and around the corner to your
left from where you just threw that switch).  You have to
get to that walkway, either by jumping from where you are
now, or you can go back to the central pillar in the slime
lake, and Run north along the bridge (activated in "Method
2", above), to jump north onto the raised walkway.
Follow the walkway north and east, and jump down into the
large, darkened area ahead of you.
Within this large room is another structure with a central
entrance; go east through that entrance and drop down into
the area ahead of you.

In front of you now you'll see 2 small pillars, or "slabs",
which block a forward path which leads to the exit switch.
Back and around the corner to your left and right are a
pair of tall elevators, or lifts.
Go around to your left, and Activate the northern-most
lift.  (Only the north lift will drop when Activated from
below; the south lift will not.)
Step off the lift into the room at the top.  When you do
this, the lift will drop again.
You'll see a switch on the wall in this room, but don't
Activate it immediately; wait until the north lift rises
all the way back up again.
Activate the switch, then quickly run back to the lift, and
it will drop.
When you get to the bottom, run to the south lift (which
will be down now, momentarily).  Ride this lift up, and
when you get to the room at the top, look for a switch.
Activate this switch to drop the 2 slabs that were blocking
your path to the exit switch.  Go back to the lift and ride
it down.
Just behind the place where the slabs stood, you'll find a
teleporter booth on the right; and the exit switch is at
the center of the east wall in the room ahead of you.

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