To get to the exit switch in level E4M7, And Hell Followed:

First, get the red key.  It's in the room that's at the
south end of the west edge of the map.  The key is sitting
on the window ledge.
Go through the red-keyed door that leads to the large
courtyard at the north end of the map.
Jump down into the green-slime river that cuts across the
middle of the courtyard (you'll find protective suits at
the base of the round central section).
Run west down the tunnel, then south to some red-keyed bars
blocking your path.
Open them, then get on the elevator that's just ahead of
you to your left.
Ride up, and Activate the skull switch in the room ahead of
you.  The walls drop, revealing another switch.
Activate that, then jump out the window ahead, to land back
down in the courtyard.
Two bridges will be rising up along the sides of the round
central structure in the slime river.
Walk to the middle of that structure, and a switch will
rise up out of the floor... it's the exit switch.  Activate
it and your gone.

To read about or download a quick demo that shows how to
find the exit switch on this level, click here.