You begin at the top of the centermost of three waterfalls, overlooking a large valley. The rivers from each waterfall converge—the namesake confluence—and continue downstream to the far end, where there's a large overhang spanning the valley. On either side of the center waterfall you'll find two doors marked with the exit symbol. A Yellow Door is off to the side. Down at the far end the river flows under a gate and beyond. To either side is a Green Door. Pick up the Yellow Key, which is in the river in front of the gate. After you step here, look at those gray columns nearby: Doors on both have opened, with a couple enemies and items inside.
Go inside the Yellow Door. In here is a short hall, and two rooms on either side visible through windows. In the left room is a covered-up teleporter, and in the right-hand room two switches. First, go straight ahead to the far end of the hall and activate the wall between the tapestries to open a hidden area with an item inside. Go to the right-hand room and hit both switches. The one on the right unblocked the teleporter in the other room. Go through this teleporter: it takes you up to the top of the nearest side waterfall outside. Collect the Green Key here. There's another teleporter up here that was unblocked by the other switch in the room, which just goes back to the first teleporter.
Now that you have the Green Key, go through either Green Door and into the main building. Both ways go to a large room with a big elliptical pool surrounded by a low wall. (The river appears to go here. If so, it must flow through a subsurface passage.) There are enemies wandering around in the pool, but the wall keeps them there and also blocks their weapons-fire.
Just inside one Green Door is the Blue Door. When you go for the Quartz Flask on the small pedestal nearby, a small room opens outside between the river and one of the Green Doors, with a Mystic Urn inside. Go across to inside the other Green Door, where there's a wooden door. You'll have seen this already if you came in here this way. There's a sizable network of rooms in here, in three sections: left, center and right, leading off the first room. Take a look down the stairway to the right: just around the corner is a gold symbol on the wall. This is a door to a hidden area with an item. Further down here is a big network of random rooms, with lots of items. You'll find a teleporter in here: this just goes back to the hall inside the Yellow Door.
Back in the first room, go to the left section. In here is a side room with a window looking into the center section. You can get rid of some of the enemies from in here. Go through the wooden door to the center section now. Look immediately to your right, behind the pedestal. The misaligned texture on the wall is a door to a hidden area with an item. Go to the far end of the center section, and hit the switch you find. Last, go to that side section across from the window, where's a Dragon Claw and ammo. After you're done in here, go back to the big room with the elliptical pool.
At the far end, two large passages have opened, both of which go up stairs and to that room inside the window. Go here and collect the Blue Key. Watch out for the ambush you get coming from the passages. When you've fought them off, take a look around both passages: in each, two areas have opened, with a lot of items.
Go now to the Blue Door. Inside, a corridor goes left and right. The way to the left goes to a room that's out of reach overhead. Look across the hall for a red tapestry on the wall: in here is a hidden area with a weapon. Go to the right, and to the end of the corridor, where you'll find three switches. First, activate the wall with the red tapestry: this is a hidden area with items. Hit the switch to the left, and go inside the door that opens nearby. There's a ton of ammo in this room. To open the door and leave, hit the switch in the back. This switch also raises a set of stairs down the corridor to the left of the Blue Door. Before heading that way, hit those other two switches and go back out to the pool. One switch lowered the wall around the pool, allowing you to get to the items. The other opened a big door to the section of river past the gate. In here is the Phoenix Rod you could see earlier.
Go up the now-raised stairs inside the Blue Door to the room above. If you go for the Inferno Orb on the red pedestal, immediately move: there's a crusher above that activates. The red tapestry on the wall is yet another hidden door to an area with items. Continue beyond up stairs that double back. On the second landing, look to the right for a marked section of the wall. This is a door to a hidden area containing Wings of Wrath. Continue on and out to the top of one of the side waterfalls, and hit the switch you find here. This opens the two exit doors down in the valley.
Go here now. Both passages lead to a small water-filled room. When you step on the blue pedestal, the exit portal is uncovered. Before leaving, look in the passage inside the left-side exit door for a gold decoration on the wall. This is a hidden door, inside which is the Map Scroll. Hit the switch, and a lift to the right with items on it lowers, which leads up to a small water-filled area. Activate the wall above the step, and it lowers, connecting back to the top of the center waterfall where you began the level. You'll now have completed your tasks and seen everything. Enter the exit portal to continue your mission.