This level has one of the most creative and unique designs in the game. From where you begin, you'll see several very long corridors in various directions, lit by flickering torches. First, thoroughly explore these corridors. There's a huge number of enemies in here, and as you clear them out and explore the area you'll fill out your automap. There's also an enormous quantity of ammo available here.
Behind where you first entered the level, you'll find a meandering passage, at the midpoint of which is the Yellow Door. Along the east/west corridor on the north side of the level, you'll see four inset areas with decorative panels which are doors. When you approach any one of them, all four will open. Inside are rooms with items. When you enter any one of these rooms, hidden doors in all four rooms will open, with Iron Liches inside. Keep an eye out while you're fighting the first Iron Lich you come upon, because the other three are probably sneaking up on you. There are additional items in the rooms where the Iron Liches were.
You're now ready to begin your main task. If you look at the automap, you'll see that the corridors divide the level into several sectors. Each of these has some sort of room or rooms, area, chamber, etc. inside, each with a different theme. You probably saw doors leading to some sectors, one sector has windows, one has ledges out of reach, etc. There's only one overall objective in this level: find the Yellow Key. Your path to the Yellow Key will involve visiting almost all of these sectors, some of them several times. First off, an unrelated bonus sector is available at any time: the narrow one to the southwest with the wood-paneled ledges. Look near each end of the south wall for the textures that are slightly different. These are hidden doors inside.
Go to the third sector from the west, along the northernmost corridor, and activate the section of wall on the north side that's slightly different, to open a door. Quickly run through the pool: a crusher above will activate, but also the column in the middle will lower, with a Bag of Holding on top. Hit the switch in the far corner.
Go to the sector to the north of the one with the windows. A door leading into here is now open. In here are three teleporters up short flights of steps, only one of which is uncovered and reachable right now. Remember where this is, because you'll need to return here. Take the teleporter, and you'll end up in a dimly-lit sector with a bunch of columns. First, activate either of the walls with the gold decorations, which are doors that will open. Step out into the corridor, then go back inside: the columns have opened, uncovering items. Collect the items quickly, because the columns will become crushers when you step under any of them. When you're done, return to the three-teleporters sector.
A second teleporter is now open here. Take this one to a new sector. In here is a labyrinth of partitions around a column. After clearing out the enemies, make your way to the center of the labyrinth. Shoot at or strike the column, and it will open, revealing a switch. (You probably already shot it open while fighting the enemies.) Hit the switch, and the door to the sector will open, but also three side areas will open, with a lot of Disciples of D'Sparil inside, so be ready for them. When you step into the largest of these side areas (the one on the east) a fourth area nearby will open, with additional enemies. When you step into the fourth area, the partitions will momentarily lower, allowing you to get to the items on top. When you're done in here, return to the three-teleporters sector.
The last teleporter is now open. Take it to a wood-paneled room with neat-looking decorations along the walls, and a lava pool with several columns. Look for and hit the switch in the corner behind where you teleported in, and the middle column in the pool will be opened. Go for the Dragon Claw that was under the column, and the sector door will be opened.
Go now to the second-to-westernmost sector. The door into here is on the west wall, near the north. In here is a long stairway leading down, plus a second long stairway adjacent to it leading up, that's out of reach. Go down the stairs, and through the teleporter, which goes to the sector with the windows. Hit the switch next to the door (it will be open later), collect the ammo, and then take the teleporter you initially landed on.
You'll be in a new sector, this one covered in lava. (You landed on another teleporter; this one just goes back to the windows sector.) You'll need to cross over to the far end of the sector. If you have a Ring of Invincibility, it's a good idea to use it here, to prevent health loss from running around on lava. Between here and the far side of the sector there are three short curving passages to the right, one after another. All three of these have doors at their ends to hidden areas, with items inside. You'll get Wings of Wrath later on; you could return here then and avoid the lava altogether while exploring those. Either way, you'll need to run over the lava this time around. There are Flame Orbs everywhere in here; try to get them as you cross over. The door at the far end exits out to the corridors.
Return to the long-stairways sector. The switch in the windows sector unblocked the stairs leading up. On the far side, you'll find a third stairway leading down, but there's nothing down there for now. Go up the center stairs, where a teleporter leads to a new sector.
You'll have your nose up to a door. Turn around, activate the wall with the blue tapestry, and take this pathway up. Every wall with a blue tapestry in here is a door. The path goes up a long stairway to a landing, up a short stairway to the left, to a second landing, and up a second long stairway to the left. Open the blue-tapestry door on the second landing for a bonus area. Among the items in here are the promised Wings of Wrath. (The blue-tapestry door in here goes to the first landing.) Go to the top of the second long stairway, and open the door here. Inside is a long, deep lava-covered room with a meandering row of tall pillars forming steps down to the far end. Be careful running down these, because Gargoyles hiding between the pillars will get in your way and might cause you to fall off. The door at the far end leads back to the second landing. Near the end of the row of pillars, there's a teleporter down by the lava. Jump down to and take this teleporter: it leads to the door of this sector, which will now be open. (Sometime, come back here and fly around down at the lava. There are Flame Orbs all over the place.)
Return to the long-stairways sector, and go down the far stairs. The now-uncovered teleporter here goes to a new sector which consists of a single elongated spiral passage. As you make your way toward the end, parts of the walls will lower, opening the area up. After you get to the end, all of the walls will have lowered and a single column will be left. Step on top of any part of the last section of wall that lowered, and that last column will lower, on top of which is the long-sought-after Yellow Key. Collect it, and a small area to the left of the door will open. Step onto it (where the Bag of Holding is) and the door will open.
Go to and open the Yellow Door. Step onto the green part of the floor, and two side areas will open, with enemies inside. After taking care of them, look in each side area and hit the switches you find. One raises a stairway to the platform; the other uncovers the exit portal. (There's a Map Scroll in one side area.)
Before leaving, there's one last sector that opened when you opened the Yellow Door. In that sector, along the triangular loop passage there's a switch. Hit it, and the center lowers, with an Iron Lich and armor. Before leaving, be sure and have a fly-around. Especially, look around on lava for Flame Orbs; plus, remember the bonus areas in the lava-covered sector if you didn't get to all three of them earlier.