Escape the 4-walled trap in Black Tower (BLACKTWR.WAD):

(Note: For a video demonstration of the walkthrough below,
click here.)

In the octagonal room where the yellow key is found, there
is a four-walled structure that is initially recessed into
the floor.  There are a couple of occasions when this
structure may rise up to surround a central teleporter.

Because of this, you may at some point teleport back into
this room from a later part of the level, and find yourself
surrounded by walls and unable to proceed.

To lower them, look for a switch in the southwest corner,
where two of the four walls meet.  Note that this switch
may be just behind a floor-to-ceiling pillar, and/or may be
high off the ground; But it can still be activated from the
side or from below.

There is also a duplicate switch on the exterior of the
structure at its southwest corner, in case it ever needs to
be lowered from the outside.