To find the exit in Doom II level 28, The Spirit World:

First, look for a small, square chamber on the west side of
the Automap.  The dominating feature in the chamber is a
central, four-sided pillar, with an animated texture that
flows around its entire surface.
(For help locating this chamber, see "FINDING THE CHAMBER",
below.)

There are 3 simple, circular diagrams on the walls of this
small chamber: A pentagram on the east wall, an incomplete
pentagram on the south wall, and a circled, inverted "V" on
the west wall.
Each of these circular diagrams is actually a switch, and
each switch can be activated by attacking it with any one
of the first 4 weapon types (fist/chainsaw, pistol, shotgun
or chaingun).

Hitting the west (Automap west) diagram opens a hidden door
to the Exit area.  (Note that you will also need a red key
to proceed through yet another door which blocks access to
the actual exit portal.)

Hitting the east diagram opens a nearby wall panel (if it's
not already open), which provides access to a passage that
leads to another part of the map.  Note that this panel can
also be opened from the opposite side, if you're coming
from the other end of the passage.

Hitting the south (middle) diagram will raise the chamber's
central pillar into the ceiling, and a rocket launcher is
revealed on the floor.

This chamber (if you haven't found it yet) can be accessed
through two routes.
The first route is via a false wall on the west side of the
Automap; the second route is through the passage which is
opened with the east-side diagram (see above).

FINDING THE CHAMBER
-------------------
One way is to first find your original starting position on
the Automap: It's a small, circular room to the north, with
passages leading north and west, and a door to the east.
On the Automap, look for the path out of that room that
leads west, and begins a sweeping, widening curve to the
south.  At the end of this path is a very large, very
irregular room, with curving grey walls, and floor like
cracked, dry mud with a few sunken lava streams.
Look on your Automap and find the very southern-most point
of this area and move to it.
Once there, face west (Automap west), and follow the grey
wall at your left a few paces clockwise around the edge of
the room, until you come to a small, squarish cul-de-sac on
your left.
The back wall of this 3-sided area is an intangible, false
wall.  Walk northwest into and through it.
Follow the short path around to the south, up a couple of
wide steps, and you'll be in that square chamber with the
designs on the walls...

To read about and/or download a demo showing two routes to
the exit chamber, and how to open it, click here.