Find the secret exit in Original-Xbox Doom II level 02,
Underhalls, which leads to level 33, Betray:

In Doom II map 02, Underhalls, go to the southeast corner
of the level (where the red key can be seen through a small
window), then look at your Automap.
There is a wide, raised ledge running west-to-east, then
north-south in a rough "7" shape along the north and east
sides of this rectangular area.
Get on this ledge and go to its southern end.  Turn west,
and you'll see a window in a structure in front of you.
Align yourself with the center of the window, put your back
against the wall behind you, and then Run to jump through
the window (use the Left-Trigger, or "Sprint" button).
Note that you won't be able to jump over any live monsters
on the ground in front of you.  If you bump into a monster
and fall, or miss the jump for any other reason, simply go
back and try again.
Once you're through the window, look for the stairway ahead
of you and to the right.  The red key is at the top, next
to a small window.
As you grab the red key, look out that window towards the
right, and you'll notice that a section of the wall outside
in the northeast corner has opened, revealing a rectangular
chamber.
Walk out the window to drop down, then walk up the nearby
steps and turn right, and head over to that newly-opened
chamber in the northeast corner.
Step inside and face the chamber's long back wall (its
north wall).  Turn left, and you'll see a narrow vertical
wall section at the northwest corner of the chamber.
Step up to that narrow west-side wall section and Activate
it like a switch.
The level will end, and you'll go to level 33, Betray.