Access the keys and the exit in Master Levels map VESPERAS:

The yellow key is in the large circular area on the west
side of the map.  Go to the entrance of this area, and the
vertical bars there will rise up as you approach.
Go past the bars and look left.  If you've already explored
this area, you should see a long horizontal opening in the
curved wall to the south, up above the slime-covered floor.
If that wall section is not already open, then step onto
the edge of the rectangular pillar next to you, and then
step back again or jump to the red teleporter pad down in
the slime.  (Crossing the edge of that pillar triggers the
opening in the wall.)
If you see the horizontal opening in the south wall, you
can WALK to it by following a very narrow ledge that runs
along the curved wall from the room's entrance.  (In PC
games this ledge is almost invisibly narrow.)
Staying close to the wall helps in navigating this ledge.
Once you get there, look for a face-like switch on the back
wall.  Activate it and the switch-panel rises up revealing
the yellow key.  Simply grab it.

Note: To get the red key, you'll need the yellow key.
The red key is in a smaller circular room in the northwest
corner of the Automap.  It's in a high chamber on the west
side of this room, and the chamber is initially hidden, but
will be revealed as you walk to the east side of the room.
Turn and walk away from this room through the east-side
doorway, and look for the floor to drop out from under you.
When you drop down, you'll see small chambers to your left
and right.  On the back wall of the right-hand (south)
chamber is a face-like switch.  Activate it, and the floor
of the high chamber to the west will lower, bringing the
red key down to ground level.

Note: To get the blue key, you'll need the red key.
The blue key is in a large rounded chamber on the north end
of the map.
Go north through the red-keyed door at the entrance to this
chamber, and drop down to the floor below.
The blue key is ahead, north, on low triangular pedestal.
To lower the pedestal, Activate a switch you'll find on the
wall behind you (just under the entrance to this chamber);
Then simply grab the key.

The exit chamber is in the square building that's in the
middle of the south side of the map (this is the building
where you first appeared on this level).  The door to the
exit chamber is inside this building, on the south wall;
You'll need the blue key to open it.
Activate the switch on the back wall of the chamber to end
the mission.

PC Doom II players can access a demo of this level that
shows how to access all keys, by clicking here.

Hope this helps. :)                                      

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