How to find the secret exit in level E2M5, Command Center
(and get to level E2M9, Fortress of Mystery):

You start this level in the middle of the north edge of the
map.  This will be the starting point for the walkthrough.

Go through the door in front of you, and turn right.
Continue on around to the right, to a long shallow stairway
that leads up to the south.
When you come out of the opening at the top of the stairs,
look to around your right for a skull-faced wooden panel in
the nearby corner.  Activate this panel and it rises up,
revealing a short stairway leading down.
Follow the stairs down, and continue along the winding path
through a couple of narrow, empty rooms.
You'll come to another wooden skull-faced panel.  Open it
and go through.
Turn right and walk a few paces, and then look for another
skull-faced panel on your right.  Go through that one, and
look to your right for a skull switch, and Activate it.
Leave this room and go right.   Continue around to the
right and look for some steps that lead up to a reddened
panel recessed into the wall.
This red panel is a false, intangible wall; you can move
right through it.  Walk through it and step on the teleport
pad you'll see there.
You'll re-appear in a small round room on the west half of
the map.  Behind you is a switch-door.  Activate it and go
through to another teleporter, and step on.
You'll re-appear in a long room at the northeast corner of
the map.
You'll see an exit door in the distance ahead of you.  Go
through that and flip the switch you'll find, and you'll
exit to level E2M9: Fortress of Mystery.

PC Doom players can access a demo of this level that shows
how to get to the secret exit by clicking here.