Walkthrough for level 07, Dead Simple:

You begin the level in a courtyard.  In the middle of the
courtyard are five wide, square pedestals: one central
pedestal surrounded by four more.  The central pedestal, a
pink cube, has an exit switch on each face, but these
switches are inaccessible until every monster on the level
is dead.

Along the courtyard walls surrounding you are 4 more
switches.  These switches each lower one of the four
remaining pedestals in the central courtyard.  Each of the
four pedestals has a goody on it, as well as a Mancubus
monster (if playing on higher difficulty levels).

Kill all the Mancubuses, and the walls surrounding you will
open up.  Then kill all the Arachnotrons that come after
you through the opened walls.
Once they're all dead, the central exit cube becomes
accessible via an emerging stair-step, and you can reach
the exit switches.

WARNING: A possible bug will occur if the last Arachnotrons
are killed in rapid succession.
The death of the last Arachnotron should cause a stair-step
to rise up around the pink exit cube at the center of the
map, allowing access.  If the last two Arachnotron deaths
overlap, it's possible that the stair-step will rise TWICE,
making it too high to climb.
If this happens, you can still get to the exit cube by
running to the center platform from the (lowered) northeast
or northwest platforms.