The majority of this level consists of outdoor and underground waterways, most with fast currents, and reservoirs. Maneuvering will be significantly easier if you use "freelook" with the mouse. You begin outside in a water-filled upper area that outflows to a larger lower area. Nearby is a water-filled channel leading to another area well out of reach above. Follow the outflow down to the lower area, where you'll find the Yellow Door, an overlooking area, and a channel leading underground.
After clearing out the enemies, continue down the underground channel. Before long you'll pass a wider area, with the Yellow Key out of reach on a water-covered ledge. Continue along the main route, and you'll pass a small side area to your right. If you go for the weapon in the alcove, a compartment behind you will open, with an enemy. Further along the channel there's a route leading down to the right, blocked by bars. At the far end of the main channel, hit the switch you come to: this lowers the ledge with the Yellow Key.
Lava rooms bonus
Return to and collect the Yellow Key, and continue outside. Along the way you'll find another side area open on your left. In here are three lava-covered rooms to the left, right and ahead. The rooms to the left and right have items, plus overlooking areas at the far ends. Both overlooks will be accessible later through secret areas, so be sure and be on the lookout for them. When you step into the center room, the door behind you closes, but the walls ahead and to the sides open, revealing a much larger room with enemies, lots of items and three teleporters. Each teleporter goes to an area you've already been to: one to the switch at the end of the channel, one to near where you began the level and one to just below the outflow. Run to and take one of the teleporters right away, step aside and wait a bit: the Weredragons back in the lava room will eventually wander into the teleporters and be sent away, usually to below the outflow, and can be fought much more easily from outside. After taking care of them, go back to the lava rooms, and hit the switch to re-enter the center room. The walls inside will have closed, and will reopen when you step into the room. In addition to items here and there on the lava, there's an item on each teleporter (you picked one up already). Keep returning to the room to collect the remaining items.
Room and trap
Go back outside, and enter the Yellow Door. Hit the switch in the room at the bottom of the zigzagging steps. This will raise the lowest step a bit, preventing exiting, and the back wall will open, releasing an ambush. When you step into the opened area, a second area across the room will open, with enemies and an item inside. The switch in the first opened area momentarily lowers the lowest step, allowing you to exit.
There's a hidden area available in here. First, note the ledge in the main room. Go to the second side area that opened, and poke around along the wall to your right: you'll find a hidden compartment with a switch. Hit the switch to momentarily lower the ledge, ride it up, and activate the back wall to open a hidden door to the overlooking area in one of the lava rooms. There's a good item here.
Reservoir and channels
Go back outside and down the channel. The route that was blocked by bars is now open. Follow it to a large underground reservoir with floating spores and many overlooking areas and additional channels. Two channels to the left are blocked by bars; to the right is the Green Door. An easy to way to clear out the enemies in here is to set off the spores. Back up right away if you do this, so the gas clouds don't impact you. More spores will spawn whenever any are destroyed.
Find and take the channel that's not blocked. Just above is a confluence with four additional channels. From the left: the first leads up and quickly ends in an opening overlooking a channel flowing down to the left. The second is the downward-flowing channel you just saw. The third and fourth both goes up and to the left, and also look down on the second channel. There are items along the channels leading up. In the fourth channel, activate the wall between the hanging skulls to open a hidden door to a long room with items inside. When you step into here, another door across the channel opens, with an Iron Lich inside. There's another hidden area available: go up the first, second or fourth channels, jump out the overlooking opening down to the second channel, and return to the confluence. A low alcove in here has opened. Step into it, and a hidden water-covered lift lowers to a small chamber with ammo and armor.
Go down the second channel. The route will branch, with the channel continuing up and a short route flowing down to the left to a dead-end with a Tome of Power. When you approach it, a compartment nearby opens, releasing an Iron Lich. There's a Map Scroll inside the compartment. Continue up the channel, which goes to one of the rooms overlooking the reservoir. In here is the Green Key.
Return to the reservoir, open the Green Door, follow this channel up to another overlooking room, and hit the switch you find. This unblocks the two remaining channels down by the reservoir. Across the room is a hanging skull: activate the wall next to it to open a door to the overlooking area in the other lava room. There are several items here. When you're done, return to the reservoir.
Both of the just-unblocked channels go up to the final overlooking area. Partway along one channel is another route leading up and away. Go this way: partway up you'll find a Mystic Urn in a nook to your left. This final channel leads up to the area overlooking the lower reservoir outside. Along the channel just inside, look for the hanging skulls and activate the wall here: this opens a hidden door to a room with a weapon, plus opens another door behind you, releasing an Iron Lich. There are Wings of Wrath in this second compartment.
Go up to the overlooking area outside and jump down to the outdoor reservoir. When you do, water-covered steps form in the channel near where you began the level, leading up to the out-of-reach area. Go up to this now-accessible area, get a good weapon ready, then step on the square with the exit symbol. The outer walls in here open, with a good-size Disciple-of-D'Sparil ambush inside. After fighting them, collect all the items inside the opened area, then go back down to where you began the level. A small area nearby has opened, with the level exit inside.