You begin in an empty room. Through the door ahead is a tall hall with a water feature, two high platforms on the left, two ahead, and one on the right. The Green Door is above the leftmost platform; two Blue Doors above the one on the right.
After clearing out the enemies, hit the switch to the left to momentarily lower the platforms ahead, and go to the next room above. In here is a ledge along most of the room, with the Yellow Door above the ledge to your left. When you step onto the green part of the floor, an area between the two lifts that led up here, plus two compartments on either side, open. Look inside the two compartments and activate the walls with the gold symbols, which are hidden doors to small areas with items including a Mystic Urn, that look into parts of the level you'll get to later. Next, step up to the decorative panel in the room above the lifts, and the ledge in front momentarily lowers, allowing you to reach the rest of the ledge.
First lava room
On the opposite side of the room from the Yellow Door, drop down the hole inside the nook in the wall. In here is a long, deep lava-filled room with rows of columns on either side. First, you'll see two switches in the nook you dropped down into. One causes a high walkway leading to the far end of the room to rise into place, plus it uncovers teleporters under the two columns at the opposite end of the room. The other switch uncovers teleporters under the two nearest columns.
Go across the walkway to the dark room at the far end and pick up the Yellow Key on the pedestal. You'll get a fair-sized Sabreclaw ambush that isn't much trouble. To exit the area, you'll need to use the teleporters down by the lava. Either of the near ones sends you back across the walkway to the nook; the ones on that end return you to the ledge in the room with the Yellow Door.
Lava, then columns
Inside the Yellow Door is a deep lava-filed room. After clearing out the enemies, look along the wall that was to your left when you first entered the room, and in an inset area you'll find a hidden door with a teleporter inside. This goes to the overlook across the lava where the Iron Lich was, where there are items. The back wall in here is a hidden door; the teleporter inside exits the overlook. Also, there are items down on the lava including a Ring of Invincibility, although you might want to come back for them later with Wings of Wrath.
The next room is partitioned by columns into inner and outer areas, and you'll see the Green key on a tall pedestal in the center. At one end of the U-shaped area outside the columns you'll find a column that's blocking a switch, although you can't actually see the switch itself. The other way along the U-shaped area has an inset area with a blue tapestry and a switch just to the right, plus another switch at the far end of this side. Hit the switch at the end, which unblocks the switch behind the column, then go to and hit that switch. Watch out for some enemies in compartments that opened along the way. The switch behind the column lowered the pedestal with the Green Key, plus it opened a hidden door at that tapestry in the inset area, which leads back to the tall hall.
Blue Key and extras
Collect the Green Key, then, before going back to the tall hall, hit the switch next to the inset area. This momentarily lowers the platform in front of the Green Door. Inside the Green Door is a short hallway with two doors to the left and one at the far end. The doors on the left go to a side area that isn't necessary to complete but will give you a weapon and some items. Inside is a smallish room with a bunch of columns that make it hard to move around. Hit the switch between the doors, and a compartment in the room will open, with a Hellstaff. When you step into the compartment you'll get a pretty nasty ambush, so have a weapon ready. Inside the compartments that opened are items including a Map Scroll.
In the big room past the far door in the hall you'll find the Blue Key on a construct. This is an obvious ambush point, and the enemies this time will be lots of Disciples of D'Sparil from from areas that open on either side and ahead. But all the Flame Orbs behind the construct, and a bunch of other items available, will go a long way in dealing with them.
When you're done in here, return to the tall hall. The platform in front of the blue doors will be intermittently raising and lowering. Enter the right-hand Blue Door: in here is a central room surrounded by a big, deep, water-filled U-shaped area with ledges along the inner (outside the central room) and outer walls. Watch out for Disciples of D'Sparil in here; plus it's possible to clear out the Gargoyles that will fly into here through the windows. Hit the two switches in here, then go back outside and through the other Blue Door, which leads to the U-shaped area. After finishing with the enemies, go up the stairs to the midpoint of the outer wall, where you'll find Wings of Wrath, and the level exit. One of those switches you hit in the central room opened the door to the room with the exit; the other lowered columns in front of the door. To get out of the water or back up to the inner ledge, go to either end and activate the section of ledge with the dark metal texture, which is a lift.
Don't exit yet: one more hidden area. Step into the room with the exit, then go back outside, and you'll see that the ledges in front of those decorative panels up on the outer walls have momentarily lowered. Ride the one up on your right (Blue Door side) but then jump off: the panel opens, revealing a hidden room with an Iron Lich just inside that's easy to get toasted by. There are items available inside including a Phoenix Rod. (The other decorative panel doesn't go anywhere.) Before leaving, remember the items down on the lava throughout the level, now that you have Wings of Wrath. Up next is the final full-size level of the episode.