The beginning task of this level is similar to the beginning MAP07 from Doom II; all the mancubi at ground level need to be killed before the rest of the level opens up. After killing them, lifts will be revealed. Go to anyone and lower it, travel along the way until you find a gap leads to the Megasphere and the yellow key. Run fast toward it (an Archvile will appear which keeps teleporting between two cages), go back again to any of the lifts and find the yellow door, open it and click the switch (instant Hell Knight will teleport).
Go back to the gap, again lower the lifts but do not go up, bridges are connected with the outer area now. Go and find a brown door, open it and clear your way down to the exit.
At the end of the level, enter the room in the immediate vicinity of the exit teleporter, but DO NOT walk into the teleporter itself. Crossing into this room causes a pair of secret rooms to open at the opposite end of the green-walled corridor.