This level, in addition to having a unique design, has a great deal of ammo and items, and you'll be able to fill out much of your inventory. This level is arranged around a very large square ring-shaped outdoor area with a wide concourse along the outside, a water-filled channel inside that, and a narrow ledge along the inner wall opposite the channel. You'll see the Green and Blue Doors along the concourse, and the Yellow Door along the inner ledge.
First, clear out enemies and collect the ammo. You'll find out the inner ledge is inaccessible for now. Don't prioritize the Ophidians on the ledge too much: you'll get a Phoenix Rod shortly and can clear them out efficiently then. You'll see several windows along the outer wall looking out into a water-filled outer region. You may have gotten the attention of Gargoyles outside these windows; also, you'll see three overlooks outside, each with a big crowd of Disciples of D'Sparil roaming around. You'll also see that these overlooks correspond to sections of wall along the concourse with the tapestries. These are indeed doors that will be opened later, releasing the Disciples of D'Sparil.
Pyramid and teleporters
After taking care of enemies, go to the corner of the area next to the Green Door, where there's a teleporter. This goes to a high overlook at the corner of a big square flooded area, with a pyramidal structure in the center swarming with Ophidians. Nevermind the Ophidians for now, and instead clear out the enemies wandering around in the water. When you jumped down from the overlook, the outermost layer of the pyramidal structure lowered. Go and stand on this lowered section: when you do, a lift in front of the overlook you came from will momentarily lower. Ride this lift back to the overlook, and take the teleporter here to return to the concourse.
A second teleporter in one of the corners along the concourse has been uncovered; it'll be to your right. Enter this one, and you'll be back in the flooded area, on a different overlook. This works the same as the previous one: when you jump down, the next layer of the pyramidal structure will lower, and when you stand on that lowered section, a lift to the second overlook will lower (as well as the first, because you entered that section too). Return to the concourse (by the teleporter on any of the overlooks you can reach), where a third teleporter has been uncovered. Repeat this process with a third overlook, section and lift, a fourth teleporter along the concourse, and the remaining overlook. Your overall goal is to reach the Yellow Key on top of the structure. On the fourth overlook is a Phoenix Rod. Now that you have this you'll be able to get rid of all those Ophidians with ease. When you jump down from this overlook, the center section with the Yellow Key will lower. Here in the flooded area, in addition to the four overlooks, you'll see a door high on one wall, with a platform in front. Stand on the center section of the structure where the Yellow Key is/was, and this platform--another lift--will lower. Ride it up and open the door: this turns out to be the Yellow Door along the narrow ledge opposite the concourse outside; only you've opened it from the inside, and the flooded area with the pyramid is in the middle of the ring-shaped main area. (This, of course, could be seen on the automap.)
Go through the Yellow Door and out onto the ledge. Be on guard: those three doors along the concourse will be open now, and all those Disciples of D'Sparil that were waiting outside on the outer overlooks will be wandering around. Don't worry too much about trying to stay on the ledge. Get the enemies out of the way first, and then go back to the ledge via the Yellow Door if you need to. Follow the ledge all the way around: there's much ammo, plus you'll find the Green Key on the far side.
Bonus outer region
Before continuing to the Green Door, there's a fabulous bonus area now available, namely the big watery region outside. Go to any of the three outer overlooks. The railings are too high to reach directly, but if you stand on top of the section of door that's sticking out above the floor, you can step diagonally from there up onto the railing. Over the edge of the railing opposite the door, down on the water, are Wings of Wrath. All three overlooks have Wings of Wrath in these locations, so you'll have lots of flying time to explore. The overlooks are the only way into and out of the outer region, so be sure to get back to an overlook before the Wings expire.
When you first fly out, go get the other two sets of Wings and of course all the items down on the water, then go outside the buildings that the Green and Blue Doors lead to, beginning with the Green-Door building. There are windows looking into these buildings; you'll see several Iron Liches inside the Green-Door one. The beauty of it is you can clear them out while they're stuck inside, by firing at them through the windows, and have them out of the way when you go there yourself later. There's a Maulotaur inside the Blue-Door building; if you fight it and are on your last set of Wings, make sure they don't expire and leave you stranded outside. (If you do get stranded, see if you have a Chaos Device before reloading your game. You should have picked one up in the previous level.)
Green and Blue areas
When you're done outside, go to and open the Green Door, and follow the corridor to the end, where you'll find the Blue Key. When you collect it, doors ahead and on either side of you will open, with ... Iron Lich carcasses waiting inside to not ambush you, plus several items including another Ring of Invincibility.
Go to and open the Blue Door. Don't forget the Maulotaur in here, if you didn't fight it through the window. Open the door with the exit symbol, hit the switch in the corner in here, and the nearby column will lower, uncovering the exit portal. (If you're after secret areas, you were actually done when you began the level. This is one of the few levels in the game without any counted secrets.