How to find the exit in Doom 64 level 8, Final Outpost:

Once you have all the keys, go through the yellow door and
work your way around the perimeter of the room, along the
edge of the huge pit.
On the side of the pit which is roughly opposite the yellow
door, look for a narrow cliff that projects out into the
pit (on the Automap, it looks somewhat like a diving board
projecting out over a pool).
Walk to the end of that projection, and jump onto the top
of the pedestal in front of you.  Keep walking forward and
look for some stairs that continue up in the same direction
on the other side of the pedestal.
These stairs (which will not appear on the Automap) lead up
and around to an exit teleporter.  But watch your step: If
you fall off, you're doomed.

Good luck! :)