You begin in what looks like a small hexagonal room, but hit the switch, and the floor lowers and you'll see you're in the middle of a big square indoor area with a neat design. On three sides you'll see windows to overlooking areas with Ophidians wandering around and firing at you. In one corner is a big cage with a huge stash of items and several Iron Liches. They may fire at you while you're busy elsewhere, so keep an eye out, and deal with them before continuing on. Also in this room, you'll find the Yellow and Green Doors in two other corners. Go to the fourth corner, and you'll find a stairway to the left leading up, and a room ahead will open with a small ambush and items.
Follow the path upstairs, and you'll come to a hall with rows of columns on either side. Go up to the far end and collect the Yellow Key on the pedestal. When you pick it up, two doors on either side will open, with a small ambush, plus a door ahead will open. Follow the path ahead, and you'll come to a lift that goes back down to the stairway that leads to the hall. Before continuing to the Yellow Door, go back up to the hall: there are three bonus areas worth seeing. First, look on the two rows of columns for two switches, one on each side. Each switch opens an area on that side, behind the columns. One has a Hellstaff. For the third bonus area, when you first came into the hall you passed two stained-glass windows at right angles to each other. Go up to where the Yellow Key was, and step into either of the side rooms where the ambush was, or the passage leading to the lift. When you do, those two windows will open, revealing a room inside. In here is a teleporter that goes to the big cage with all the items. (Take care of the Iron Liches before coming in here, if you didn't yet.) When you're done in here, take the teleporter in the corner to go back to the room behind the windows.
Not quite watery
Go to and open the Yellow Door. In the room in here you'll find four switches on a column, each of which opens one of four doors in the room. To make things easier, hit the switches one at a time. There are Ophidians and other enemies all over the place in here, and it's better to deal with a small group at a time than a big crowd all at once. One of these doors leads to a second room, where you'll see the Green Key in a pool that's covered up and unreachable. Continue past here, through the door and to a long hall with nooks along the walls on either side. There's a bonus area here: go to the nook that's deeper than the rest. A door here will open, and the passage inside goes up to one of the rooms overlooking the main central area, where there are a lot of items. Take the teleporter, and activate the wall ahead of you to open a door going back into the central area.
Back in the hall, go to the far end and follow the passage around to where it ends, and hit the switch. Go back to the hall right away: when you hit the switch you'll get a pretty bad Disciple-of-D'Sparil ambush on either side of you, and there's not a lot of room to move. In the hall, watch out for enemies that have been let out of compartments inside the nooks. After you get the enemies taken care of, go back to and collect the now-reachable Green Key. Several more doors will open when you pick it up, connecting some of the dead-end rooms and opening another nearby area with an item.
Go to and open either or both Green Doors. You'll find two stairways leading up, one to each of the remaining two overlooking rooms. First, explore the rooms on the ground level, where you'll find items including a Map Scroll. In one overlooking room you'll find a closed door with the exit symbol. Go to the other overlooking room and hit the switch to open the exit door, and step into the exit portal when you're done. (Those of you who are fans of flying will have to do without for now: this is one of the few levels that doesn't have Wings of Wrath.)