Inaccessible room in Halls of the Damned:

On the east side of the map is a locked, blue-keyed door.
(This is one of three locked doors in a row, leading east
towards the exit chamber.)

Just to the right of this blue door is another, unlocked
door.  Open it, and walk a few paces south, to where you'll
find an exploded corpse on the floor against the wall.

There is a secret panel in the wall at this location, and
behind it is a small chamber containing some monsters (the
number varies, depending on the Skill level).

In the original PC version of the map, this door would open
when the player goes to retrieve the yellow key.  This door
cannot be opened in Doom games on Sega 32X, Atari Jaguar,
3DO and Nintendo GameBoy Advance.

However, in those versions of Doom, rocket explosions can
penetrate walls, and that can allow the player to kill some
or all of the monsters within the closed chamber.