To access all keys and the exit on level 18, The Courtyard:

When you first appear on this map, you'll be inside a tiny
room.  The exit is behind you, but you need a blue key to
open it.
To get out of this room, shoot the door in front of you
with your pistol (or use your fist, chainsaw, shotgun or
chaingun), and it will open.
The yellow key is on the ground in the northwest corner of
the main courtyard.
Use the yellow key to enter the southeast door on the big
diagonal building on the north end of the courtyard.  Step
on the teleporter just inside the door, and you'll be sent
to an 6-sided room in the northeast corner of the map.
The blue key is in there, on the floor among many blue
health potions.
Once you get the blue key, Activate a switch you'll find on
one of the pedestals in the west side of that room.  Go
north up the steps to the teleporter, and you'll be sent
back to the building on the north end of the map.
Go south to leave the main courtyard, then west to get back
to the level's starting point, and open the exit door.

Done!

PC Doom 2 players can download a demo of this level showing
how to access all keys and the exit, by clicking here.