Inaccessible/trapped monsters in classic Doom/Ultimate Doom
level Unruly Evil:

The exit for this level is located at the north end of the
map.  There are also 2 small chambers here: one to the left
of the "EXIT" sign, and one to the right.

These chambers contain monsters (Spectre Demons at low
skill levels, Cacodemons at higher skill levels), and are
supposed to open automatically when the player activates
switches located nearby.

When playing at the 2 lowest Skills, however, the chambers
can remain closed, leaving the Spectres trapped inside.

To get a 100% Kills score at the 2 lowest skill levels, use
a rocket launcher to attack the south wall of each chamber
(aim slightly left of the wall's center) before activating
the switches that open the exit.
The rocket explosions will penetrate the wall, injuring or
killing the Spectres inside.  (This is one of the uncommon
areas in PC and PSX Doom where this trick works reliably.)

If the Spectres are killed, the chambers will actually open
when the switches are activated; They only remain closed if
the Spectres are still alive.

Note that there are other places on this map where hidden
enemies may go unnoticed during the game; but those areas
will always be accessible to a player at any skill level.

PC Doom players can click here to access a demo that shows
an example of killing the trapped Spectres on this map.