How to get up into the large blue-floored room in PC Final
Doom TNT level 20, Central Processing:

This room is located near the middle of the north edge of
the map.  When it is first revealed, its floor is too high
to step up onto.

Look for a long open hallway nearby to the south.  When you
step into this hallway, a staircase emerges from its floor,
leading upwards to the south.

Walk to top of the stairs.  As you get near the top, you'll
cross an invisible "tripwire" that will cause the floor of
the large room to drop slightly, and you'll be able to step
up onto it.

(Note that some Doom-engine "source ports", such as ZDoom,
may fail to drop the floor when you cross the tripwire.  In
that case try the original DOS games, or Doom 95, or a more
compatible source port such as PrBoom.)

To read about and/or download a demo that shows how to get
into the blue-floored room, click here.