Inaccessible secret room in level 27, Monster Condo:

The secret area most easily missed is a small room in the
middle of the western edge of the Automap.  Unfortunately
the room is automatically sealed off about 30 seconds after
the player begins the level... and once sealed, it is no
longer accessible without using the "walk-through-walls" or
"no clipping" cheat code.  (For more information on cheat
codes, click here.)

To get to the secret room without cheating, restart the
level, then run to the secret area.  Here's how to get
there from the single-player starting position:

Directly ahead of you in the far distance is a doorway you
must go through in order to get to the secret area.
Between you and this doorway is an oblong pit with green
slime at the bottom.  On the eastern side of the pit is the
bottom of a stairway that leads west, curving up and around
to a column near the center of the pit.  Walk up the stairs
until you're at the point of the stairway which is closest
to the northern edge of the pit.  (On the Automap this will
be the midpoint of the stairway's curve.)
Look north and you'll see a cluster of skulls on the floor.
Back up a little to get a running start, then Run to jump
from the stairway to the cluster of skulls.  (Remember that
in PC Doom games, "Run" is typically enabled by holding the
Shift key down.)
Now head for the door, which is to the northwest of the
skulls.  When you walk through this door you'll see a
partition ahead of you, with a giant cut-out arrow pointing
left.  Turn left and the secret area will be ahead of you.
(If you're looking at a blank brown wall, then you arrived
too late... If you see ammo items scattered on the floor
there, you made it; grab while you can.)

To read about and/or download a demo that shows how to get
to this secret area, click here.