Inaccessible monsters/areas in PC TNT level 15, Dead Zone:

There is an area on this level where one or more monsters
are trapped.  Subsequently, a player may never encounter or
be able to kill these monsters during a normal game.
This area is roughly lens-shaped, and is located just north
of the square, stone bunker on the eastern edge of the map.

The number and type of monsters located there will vary,
depending on the selected Skill level:

Skill 1: 4 Shotgun Guys
SKill 2: 4 Shotgun Guys
Skill 3: 1 Cacodemon
Skill 4: 1 Cacodemon, 1 Pain Elemental
Skill 5: 1 Cacodemon, 1 Pain Elemental

In all cases, these monsters are flagged as "deaf", and so
can only activate if they can see a player (or are damaged
by an attack).

A player may still be able to get a Kills score of 100% (or
more), by re-killing any monsters that are resurrected by
Arch-Viles.  (Arch-Viles appear at Skills 4 and 5.)

Note that there are other areas of this map which are only
accessible during Deathmatch games.  These include small
rooms which are located to either side of the stone bunkers
on the north, east, south, and west edges of the map.
(When a Deathmatch game begins, each player first appears
inside one of these small rooms.  When any player steps
away from a starting point, walls open up in each of the
bunkers.  This releases the players, and also allows access
to teleporters in each bunker.)