Enter the cavern to the east. Go into the tunnels at the north and south, and flip the switches in each one. Go to the east, jump to the lower floor, run up the stairs, and step on the demonic face panel. An alcove will open to the south; go in, and take the red key. Take the lift back up, and return to the start of the level. Head down to the lower cavern, and open the red door. Dash south through the grassy area, and head through the hallway to the east. Go up the staircase, and then enter the northeast passageway. At the end of the passage, go to the east of the pillar, and hit the switch.
Other points of interest
In the hallway of gargoyle faces, one of them, in the southwest of the hall, does not appear on the automap. Push on it, and the floor in front of it will lower. Take the lift up, and then walk through it to find a soul sphere. (sector 165)