You begin in a tall courtyard with a network of stairways bordered by tall columns. You'll see the Yellow key on top of a tall red pillar. Go up the stairs. Just before the first landing, look for a small pit to your right. There's a Dragon Claw here; hit the switch to leave. Turn left at the landing, follow this path up and to the end, jump to the red pillar and collect the Yellow Key. After getting it, go back up, turn right at the landing, and follow this path up and inside.
Just inside to your right, there's a switch. Hit this, and the stream ahead of you will lower, revealing a doorway to the right. There's a room in here with a couple items. (You might have been fired on by the Weredragons in here while outside earlier.) The switch by where you came in lowers the stream/lift again. But before leaving, look to the left of the window for a section of wall with irregular-rock texture instead of the square-cut rock like everywhere else. This is a hidden door leading outside to the ledge where the Weredragon was. (This door only opens from the inside.)
Go back up to the stream, jump down and follow the river downstream, and drop down to the lowest level. To the right is the Yellow Door, the river ahead is blocked by a stone door, and to the left is a passage to a teleporter that goes back up to the stream. Open the Yellow Door. This goes outside to a ledge overlooking a large, deep area covered over by lava. On a plateau is the Blue Key. Several doors on the far side lead from here. On the ledge, look to your right and hit the switch. This opens the stone door leading downstream, and also that stone door just upstream from the Yellow Door. Inside the door upstream is a meandering passage leading to that side room looking out over the lava area, where the Weredragons were shooting at you earlier.
(There's a bonus area down by the lava, that's also your exit from here if you fall in. It's not the dark doorway near the wooden door, but further left, behind the plateau opposite the teleporter; look on the automap for it. But nevermind this for now; it's lava-filled and you can fly here later. In fact, you might see the Wings of Wrath that are in that tall room up on the wall past the plateau.)
Go back to the river and downstream through the opened door. This goes down to a flooded room that looks like a dead-end. Go to the narrow part at the far end: this causes the walls on both sides to lower, making the room much larger. (The ambush is only a minor one.) Step into the low nook: this opens a door, leading further downstream through a tunnel. There's fast current where the water texture turns to the "waterfall" look. This drops down again to a flooded hall or indoor reservoir of sorts.
Before jumping down, there's a hard-to-reach bonus area available here. Just before the dropoff, look for a switch to your right, and try to hit it. The switch is difficult to hit because of the current, so don't worry if you fall in and miss it. You'll be able to get back upstream later and try again. (Use your "run" key, and strafe right. You'll have a much easier time of it if you use "freelook" with the mouse.) If you do succeed in hitting it, jump down to the reservoir and you'll see eight teleporters, four on each side. Quickly go to and step into any one of these teleporters, and you'll be in an upper area along the right-hand side of the reservoir. (All eight teleporters go up to here, so just take the closest one the Iron Lich isn't blocking.) Up here is a doorway to a few rooms with several items. Down a curving passage, you'll come to a wooden door. This is a back way to where the Green Key is. (You can continue from here, but for the sake of simplicity, these directions assume you'll have returned to the reservoir and are continuing by the main route.)
In the reservoir, defeat the Iron Lich and collect all the items. (You'll see teleporters out of reach on top of pillars. These pillars are momentarily lowered when you hit the bonus-area switch, allowing you to get to the teleporters.) Look around to the left and behind that big decorative panel at the downstream end, and follow this passage up and then down to a long flooded corridor.
At the far end (to the left) of this corridor is the Green Door. First, open the door to your right: this goes back to the reservoir. Now, go about halfway down the corridor and you'll come to a doorway on your right leading to a flooded side room. Go to the left and to the far end. Find and hit the switch to your right (enjoy the Mystic Urn). For a side area, go down that curving passage where the texture is different. A door at the end opens, leading out to the lava area behind the Blue-Key plateau. Earlier, you might have noticed that the plateau would be out of reach from the walkway leading out to it. The switch you just hit raised a set of steps to the plateau, so when you get to the walkway you'll be able to reach the Blue Key. (The passage you're in doesn't really serve any purpose beyond allowing you to see that the steps are now in place. That bonus area is closer of course, but fly there with the Wings of Wrath instead: it's full of lava.)
Go back to near the doorway from the flooded corridor, and go to the tall part of this side room. You'll see a doorway up on the wall, and a pillar in front of it. When you step into the tall section of the room, the pillar will lower momentarily, allowing you to get up to the doorway. If you miss it, leave and then return to the tall section, and it will lower again. In this upper area, collect the Green Key just ahead. There's a weapon in the small room down the stairs to the left. If you got into the bonus area at the reservoir, that back way led to this upper area.
Go to the far end of the corridor and enter the Green Door. This goes to the right and up into a room with a pool and platform, the Blue Door and another door in the corner to the right. Along the stairway leading up to here, try activating those black/silver panels with the robed figures. One is a hidden door. When you go for the item inside, the other panels will open, with enemies and several items inside, including a Map Scroll. On either side of the pool and platform are two more of these panels. Activate the one on the left for a hidden teleporter. This goes to a small room with a Phoenix Rod and armor. Behind the armor is a door leading to the room behind the stream/lift near where you began the level. Behind you is a teleporter that goes back to the room with the Blue Door, by means of a room behind that other panel near the pool and platform.
Go out the door in the corner: this goes outside to the walkway in the lava area to the Blue-Key plateau. Go up the steps you raised earlier, and get the Blue Key. The teleporter goes back to the Weredragon-ledge near where you began the level. While you're on the plateau, jump off and into the nearby room in the cliff face, where you'll find Wings of Wrath. If you're ready to do a fly-around, you can go to the lava-filled bonus area now. Inside there's a Ring of Invincibility, a Hellstaff, loads of Runes and more. The teleporter goes back to near where you began the level. Go through the teleporter: there's one more area to see back this way. Go back to the room inside the stone door upstream from the Yellow Door. There's another room open in here now, with several items.
When you're done, go to and open the Blue Door, and then to the exit room and portal inside.