You begin in a small side room next to a larger room, packed with Golems, with terraces of sorts, a blue-paneled overlooking area swarming with ghostly Golems that fire the flaming skulls, the Blue Door, and another door to your left.
Where you first entered the level, there was a secret area available directly behind you. Activate the wall at the back of the nook to open a hidden door, and then the dark-paneled wall inside to lower a lift. There will be a lot of Undead Warriors in close quarters, so have an appropriate weapon ready. The lift goes up to a small window overlooking a dark-paneled flooded room you'll get to before long. You can take care of some of the enemies ahead of time, plus there are items. If you didn't clear out the Golems in the Blue-Door room first, they'll be swarming into the side room so have a fast-firing weapon ready when you ride the lift back down. But it's little extra trouble to fight them all at once like this.
Teleporters, tunnels and more
Go to the door opposite the Blue Door. In here, to either side, are dark tunnels filled with icky water, blocked by bars, one of which is marked with Yellow Door pylons. Go through the door directly ahead: in here is a room with an elaborate water feature, and two more doors on either side of it. Both doors connect to a second room overhead; inside one door is a lift leading up, the other is the return route. When you step inside the return-route door, the stained-glass panel behind the water feature opens, with an item inside. When you step inside the compartment, two more areas in the room open, with a few enemies inside.
Take the lift up to the next room. In here is a teleporter on a low pedestal, and a second identical pedestal nearby. Take the teleporter to the dark icky-water-filled room you saw from the secret area behind where you began the level. You'll be on an overlooking platform at one end of the room. You'll see the Yellow Key on a pillar. Jump down and run to the other end of the room, and a lift will lower. Ride it up, step off onto the platform behind it, and walkways around most of the room will slowly rise into place. (These are where the Time Bombs of the Ancients are.) Go around to where the barred-off window is (there's a Dragon Claw here), and the Yellow-Key pillar will lower.
Jump down and pick up the Yellow Key. When you do, bars blocking a flooded tunnel leading away will open, allowing you to exit. But also, two compartments in the room, plus some compartments back in the room above the water-feature room, will open, releasing a bunch of Disciples of D'Sparil that will get into teleporters and infest the area. Inside the unblocked tunnel is a teleporter off to the left that goes back to above the water-feature room. The bars further down will open when you approach, connecting back to outside the water-feature room. Before continuing, go through the rooms you were in recently and clear out all the Disciples of D'Sparil wandering around so they don't get in the way later. There are items, including an Inferno Orb, within the various compartments. Above the water-feature room, the wall behind where the Ethereal Crossbow was opened, with a Mystic Urn inside. Also, go back to the first room where the Blue Door is. Two areas in here have also opened, releasing a bunch of Gargoyles. If you step into either area, the back walls of both will open, releasing additional Gargoyles. But there are also more items available.
Go now to the Yellow-Door pylons. The center bar is the Yellow Door. Follow the flooded tunnel past a set of bars marked with the Green Door pylons to the right, to a long tall room with six stained-glass windows at floor level, four overlooks on the sides, a throne of sorts at one end, and a pair of curving stairways leading to doors at the other end. You'll notice you entered through the "see-through" side of a one-way stained-glass window. Clear out the enemies, then take note of the left-side window opposite the side of the room you entered from. Go to the right-hand window on the far side, find and hit the switch in the window frame, then go to the left-side window. The switch opened it, revealing a one-way window just inside. (This window will have a different texture.) Behind the window is a dark room with a Map Scroll and four teleporters, each of which goes to one of the overlooks. Step through the window and into the room, but then go back out: compartments opened behind the four overlooks, releasing quite a few enemies. After clearing them out, you can teleport to the overlooks without being accosted.
There's another bonus in here: get weapons and items ready, and step up onto the throne where the Hellstaff is. When you do, that dark-metal construct between the stairways at the other end of the room opens, revealing another overlook with a Maulotaur waiting. After defeating it, stand on the back of the throne and activate the wall behind it to open a hidden door to a teleporter. It leads to the Maulotaur overlook, where you'll find a Ring of Invincibility. Activate the wall at the back of the overlook to open a hidden door connecting to that short corridor between the doors above the stairways.
Go to the short corridor inside the doors above the stairways, and through the door at the back. This goes to a small, tall room. The entrance to the room will be sealed off by a section of ledge when you step up to the switch on the far wall. Hit the switch to lower a pair of lifts on either side. When you step onto either lift, two overlooks above the lifts will open, with enemies but also a lot of items inside. Take a lift up to the overlooks, where you'll find the Green Key. Look for and hit the switch inside one of the overlooks, and the ledge blocking the door will lower, allowing you to exit.
Go back out to the flooded tunnel, and open the Green Door/bars. Inside is a lift that goes up to the blue-paneled room you saw when you began the level. Look around the column in the area that overlooks the Blue-Door room, where you'll find Wings of Wrath. Step onto the water feature where the Phoenix Rod is, and compartments will open, releasing Disciples of D'Sparil. Step into one of them, and another area opens with an Iron Lich inside. After fighting them, find and hit the switch where the Iron Lich was. The column that the Wings of Wrath were behind will lower, with the Blue Key on top. A couple columns by the side room where you began the level will also lower, releasing some Disciples of D'Sparil. There are Greater Runes here.
Open the Blue Door, which goes outside to the namesake courtyard. There's a massive horde of ghostly Golems and their flaming skulls, plus a slow wind continually blows across the area, interfering with your weapons. Use weapons that impact instantly once you get most of the horde cleared out, for improved aim when fighting the remaining ones. When you step onto the central area where the armor is or into the pool at the far end, two large areas will open, releasing additional ghostly Golems. When you're done, find and hit the switch in one of the side areas that opened, then take either set of stairs over by the pool to the small reservoir beyond. The switch you recently hit opened a barrier blocking access upstream. In there, take the stairs and passageway up and back alongside the reservoir, and hit the switch you climb up to. This opens a hidden door on the other side of the waterway, next to the opened barrier. It's out of reach from the water but you can see where you can jump over from the passage on this side. Inside the door is the level exit.