The overall layout of this level is a hub area with four quadrants in the compass directions, that you'll be successively gaining access to. You begin in a large, mostly-empty dark-paneled room. First, take the doorway to your right: it crosses a corridor that dead-ends either direction, on the other side of which is a doorway to a large, deep, octagonal lava-covered area with a pillar in the center. This is the level hub; you'll see the Yellow Key on the pillar. You'll also see zigzag-shaped walkways down by the lava that lead in each of the four directions, and doorways high on the other walls that the walkways line up with. Step or lean out of the doorway--it's OK if you fall--and the walkway below will slowly rise into place, allowing you to reach the central pillar. Get the Yellow Key, then return to the big square room.
In here you'll see a convoluted trail of Time Bombs of the Ancients leading across the room. This is supposed to be a puzzle, the intended way being to follow the trail of items. When you step onto the first square, the next square on the intended path will turn dark. The rest of the area is divided up into comparably-sized squares, each of which will become a crusher when you step into it. If you like, you can repeatedly run across the room along each row of squares to activate all of the crushers, that way you'll see where they are.
One of the squares along the path, where it goes along the left-hand wall, will occasionally flash after you walk by it. Activate the wall here to open a hidden door, and follow this path to the end, where you'll find a big cage with several Iron Liches inside. This cage is actually a staging area for what would have been a substantial ambush you would have triggered later. By eliminating the Iron Liches now, you'll save yourself a major headache.
Back in the large room with the squares, go through the doorway at the back and you'll find the Yellow, Green and Blue Doors. Open the Yellow Door, hit the switch inside, and return to the hub. The zigzag path to the west quadrant is now in place. Inside this doorway you'll find another section of corridor like the one inside the doorway to the north. The other two quadrants have these too: as you complete areas, these will be connected, forming a loop that encircles the level outside the hub.
Past the corridor, you'll come to a large room with a big pool of icky-water down to the right, past which is an overlooking area flanked by two steep stairways leading up. In the middle of the water, among the spores, you'll see the Green Key on a tall pedestal. First, go to that side room with the windows on your left, and hit the switch on the back wall. This lowers the pedestal with the Green Key. Jump down to the water and collect the Green Key. When you run onto or off of what was the pedestal, a lift to the overlook across the water will momentarily lower. Either now or sometime before you leave the area, go up both of those steep stairways. At the top of each is a small room with an item.
Ride the lift up to the overlooking area. A big hidden platform at the back of the overlook, littered with Iron Lich carcasses, will be slowly lowering. This was the big ambush you put the kibosh on by finding that hidden window earlier. On both sides of the overlook are long corridors that lead back across the water; hidden doors at the far end of each will now be open.
Your main task here is completed, but there's a bonus area. Return to the side room with windows. When you stepped onto the Iron Lich platform, two doors in here opened. (If you don't see the doors, go step on the Iron Lich platform and return here.) These two doors are at the ends of a U-shaped passage. At its midpoint you'll find a Firemace, allowing you to complete your arsenal, and a stairway leading up and left. Get your weapons ready: there's a Maulotaur up there. You'll see that the room with the Maulotaur has a window that overlooks another room; you'll get to that room later. You can clear out the enemies in there now, plus the Maulotaur that was in here won't be able to pester you later. There's a Map Scroll hiding behind the barrels.
A new area along the encircling corridors has now opened, connecting the west and north sections. Three additional areas along the corridor that are opened later will have the same style of design. Look along the walls for a section that has a large gold decoration instead of a small one: this is a hidden door. This goes to a smallish convoluted area with several items.
Go back to and open the Green Door, hit the switch inside, and return to the hub. The path to the east quadrant is now in place. Go in here and down either stairway; they both lead to a large water-filled room with a bunch of wooden columns. One column has a gap in it well out of reach, with the Blue Key in the gap. Several windows look into a darkened passage that follows the outer edge of the room. Find the doorway leading into the passage: when you step into it, all of the windows will convert into doorways.
First, a bonus area: along the dark passage you'll find a Tome of Power on a high shelf. Activate the wall across from this (note the misaligned texture) and a hidden door will open. A stairway in here goes up to the left, over a low dropoff, and continues up beyond. High on the wall to the left and just past the dropoff is a window hidden from this side; an enemy inside will probably be firing at you. (You'll get to the room behind there later.) Jump over the dropoff, and a compartment on your right will open, with a minor ambush. Go up the stairs, follow the passage to the end and activate the wall on your right: this is a hidden route back to the room where you began the level. When you cross the highest step that led up here, a lift in front of the dropoff will momentarily lower, allowing you to return to the dark passage that way and not have to take the long way around.
Back in the dark passage, you'll see a section of wall with gold decorations. This is a door you need to open. First, look for and hit the switch along the passage, then go to the now-lowered shelf with the Tome of Power. Step onto the shelf, and the door will open. Hit the switch at the end of this wood-paneled room: the column with the Blue Key has now lowered, allowing you to reach it.
(You'll have seen what looks like a big door at one end of the dark passage, that doesn't seem to open. It opens from the other side.)
A new area along the encircling corridors has now opened, connecting the east and north sections. In here, activate the wall with the large gold decoration, and the others like it you find: these are all hidden doors. In here you'll find that room inside the hidden window, and a Ring of Invincibility.
Return to and open the Blue Door, hit the switch inside, and go to the hub. The final path, to the south quadrant, is now in place. In the south quadrant is a room with a Phoenix Rod in a center sunken section. This is one of those things that has "Ambush" written all over it. Step down to the Phoenix Rod, and areas to the left, right and ahead will open. You'll get a few Disciples of D'Sparil from the left. What you won't get are a crowd of Disciples of D'Sparil from the right, and a Maulotaur shooting at you through a window. This is because that area is the one you were able to clear out ahead of time, while in the west quadrant. Also, the remaining two areas along the encircling corridor have opened. You might want to take care of the enemies in these areas, so they don't sneak up on you later. In the area to the southeast, watch out for an ambush behind you when you open the door with the large gold symbol. In the southwest area are Wings of Wrath.
Back in the Phoenix-Rod room, you'll see a door with the exit symbol behind two wooden columns in the section that opened ahead. In the left and right sections are switches: each of these moves one of the columns, allowing you to reach the door and exit portal inside. Before leaving, there are a couple more bonus areas. Step into the room with the exit portal, then back out: a big door in the left section has momentarily opened. Follow this path, and you'll come to four large doors in series that open when you approach them, the last of which is that door in the dark passage by the water-filled room. Between two of these doors a narrow passage will have opened, leading to a room with a partition made of tall torches. Be very careful if you use your Phoenix Rod to fight the Iron Lich behind the partition: most of the time you'll be close enough to take backsplash damage. In here is another Ring of Invincibility.
At this point you should have seen everything. When you're finished, return to the exit portal to continue your mission.