This level, the final full-sized one in this episode, is a large, fairly random and convoluted but interesting complex of rooms and areas. You begin on a tall stone construct in a very large lava-covered area. A pathway from here leads to the far side of the area, where there's the main entrance and stairway leading into the complex.
Down at the end of the path, instead of jumping across to the entrance, look off to your right for another area down at lava-level. Jump down here, find and hit the switch. This raises a platform in that low section by the entrance, where the Tome of Power is. You can run over there across the lava to your left, but look to your right instead. You'll see a low platform next to the stone construct, over the edge from where you entered the level. Run over here, and hit the switch on the column to lower a lift that goes up the construct. Take the lift up, but before leaving, jump down onto the column with the switch. Look on the automap if you're not sure where to jump to. There's a Shadowsphere on the column. (If you look at the area on the automap, you'll see two additional doorways in the outer wall, each obscured by another section of wall in front of it. One isn't far from here, the other near the wide doorway. Nevermind these for now; you can get to them from inside later.)
Go back to the entrance, and up and inside. You'll find the Green Door to your right. You can clear out enemies inside through that small window nearby. Go to the wooden door that's to your left from the main entrance, and through here to a short hallway with a door ahead and one to the right. Inside the right-hand door is a randomly-shaped area and the Yellow Door. Go through the other door in the hallway to a room with an assortment of wooden pillars at different heights. To get to the ones too high to climb on, run onto them from a nearby taller pillar that you were able to climb to. On top of one pillar is the Yellow Key. The switch in this room just lowers one of the pillars, making the key slightly easier to reach. After you get it, go back to the Yellow Door, hit the switch inside, and return to the wooden-pillars room. The switch you just hit opened another door in here.
Several great halls
Past the now-opened door in the wooden-pillars room is a long hall with a big stairway leading down. On either side of the hall above the stairway are side rooms. In one of the side rooms on the left is a Map Scroll. In the other left-side room is the Green Key out of reach on a pedestal. Go over to the dark-paneled side room to the right of the stairway. Go up to the platform across the room (it will momentarily lower when you approach it), hit the switch, and go back across the stairway to the room with the Green Key. The switch lowered the pedestal, allowing you to reach the key.
You can go back to the Green Door right away, or explore further here. Downstairs is a second long hall, with high ledges along both sides and the Blue Door at the far end. First, go over to the Blue Door and watch the ends of the ledges for lifts that lower intermittently. Take the lift to the ledge on the stairway side (south side if you look at the automap) and go through the doorway on your left. This goes back to the dark-paneled room. When you went through the doorway, another door opposite the room opened, leading to a bonus area. If you go downstairs in here you'll find a window looking out over a large hall with red columns, and an Iron Lich standing watch. You can take care of the Iron Lich now, along with most of the enemies in the hall.
Go back to the downstairs hall. You've probably noticed the low ledge to the left of where you entered, and the decorative panel behind it that might as well have "Secret Door" written all over it. It is, of course: to get to the ledge, go to either of the tall side ledges, and walk off the end to drop down to the lower ledge. Make sure you've reached walking speed and don't run, and you'll be at just the right speed to not hit the upper section of wall or be short of the ledge. Inside the panel is a compartment with a few items.
There's one more bonus area here. In the downstairs hall, go up to the ledge opposite the stairway side (north side on the automap). There's a weapon at the far end. Go to the passage nearby, which drops down and goes around a corner. You'll find Wings of Wrath here. Beyond is one of the two small doorways in the large lava area, specifically the one near the main entrance. (Look on the automap to see where you are. You'll also see that the downstairs hall back inside is "below grade" compared to the lava outside.) You'll find Wings of Wrath just outside. When you walk up to the doorway, a lift behind you will activate, leading back up to the ledge. Hold off using the Wings here: there's another lava-filled bonus area you'll get to later that you'll want to save them for.
Green Door area
When you're finished exploring in this area, head back to the room inside the main entrance. In the wooden-pillars room, another door will have opened, going to a storage area with items, that connects back to the room inside the main entrance. Past the Green Door there's a series of three rooms, at the end of which you'll find the Blue Key. Nearby are windows looking into a lava-filled cave with passages on either end, and a weapon on a platform. Go back to the Green Door, and you'll find a nook to the right that opened when you picked up the Blue Key. Hit the switch inside, and a passage nearby will open, leading down to the lava-filled cave. This is where you'll want the Wings of Wrath; they'll save a lot of running around on lava. The other passage from the cave leads back outside, through the obscured doorway near where you started the level. If you didn't go to that other area by the downstairs hall with the Wings yet, you can get them now, fly over from there and get to the lava-filled cave from outside. But open the door first, because it can't be opened from the inside.
When you're done here, go to the downstairs hall and enter the Blue Door. Inside is the hall with the red columns. Go up the blue steps and hit the switch. This opens a door opposite the hall. In here is an item, the exit door and exit portal. Up next is the episode finale. (One of the secret areas doesn't always want to register on the counter. If you see by means of the Map Scroll that you've been to every area, you can disregard it.)