You begin in a ring-shaped dark-paneled room. Look along the inside wall for where the texture is different (on the south side), and activate the wall here to open a hidden door. Inside here, step on the pedestal with the weapon, and several areas in here and outside in the passage will open. Back outside, watch out for a couple of Weredragons wandering around. Step into any of the now-opened areas in the corners (with the green floor; there are also items available) and a door on the south side of the room (opposite the hidden door) will open, leading further into the level.
The next room has two terraces, one above the other. After clearing out the enemies, find and hit the switch. If you want, take a look up the nearby stairs. This is a small area overlooking a big room with a lava pool and construct. Getting into here is your main goal for now, but it's not accessible yet. Go back to the ring-shaped room: the switch opened a second door in here, to your right (east). Through here, you'll be at the midpoint of a long hall with a platform out of reach at each end. (While you're fighting the Weredragons at one end, remember not to get pummeled in the back by the ones at the other end.) Look in the inset area opposite the door and hit the switch here: stairs to one end of the hall will form. (This isn't the namesake Great Stair; you'll get to that quite a bit later.)
Go up the stairs. Up here you'll find the Yellow Door and a doorway out to the lower of the two terraces. Before continuing, look for and stand in front of the blue and red tapestry on the wall. Activate this wall, and a lift will lower, leading to a secret area with items. There's an enemy in here, so have a weapon ready.
Go out to the lower terrace. When you do, a door at the opposite end will open. This leads down to the big room with the lava feature. Watch your back while fighting the enemies: there are Ophidians all along the overlooking areas surrounding the room. Also, it's easier to attack the enemies in the corners by getting behind those pedestal-barriers and up close, although you'll get pounded on a bit. There's a side room to the right, with the Yellow Key on a tall pillar. After clearing out the enemies, jump down to the lava pool. There are narrow ledges down here that you can walk along. After collecting the Runes, cross over to the construct, activate the wall with the red eyes, and take the teleporter inside here. You'll be standing on the pillar where the Yellow Key is. Get a weapon ready: when you move, you'll pick up the Yellow Key, but a large area nearby will open, with several Iron Liches inside. There are several items in here too; remember to get them after fighting the enemies. Near here, a door going back to the ring-shaped room will have opened.
Go to and open the Yellow Door. There are two ways to go in here; the door to the right goes to the upper terrace, where you'll find the Green Key out of reach on a ledge. Back inside the Yellow door, step up to the black pillar ahead, which is a lift that will lower. Take this up, to a second identical lift, and follow this route. You'll come to a long hall blocked by stained-glass columns. Shoot at the columns to open them as you come to them (you'll pretty much do this by default as you're fighting the enemies they uncover) and go to the other end of the hall. There are two paths here. First, look to your right and hit the switch you find. (There's a weapon here too.) Take the other route, which goes down a stairway and to the overlooks around the lava-feature room. At the bottom of the stairs, take a look at the short path to your right. When you hit the switch, a lift here lowered. This is the way you need to go, but first, go along the overlook to the far end, where you'll find Wings of Wrath and other items. Note the gold symbol on the wall: this is a door to a secret area that will open later on, so remember to come back here for it. Go back to the bottom of the stairs and take the lift here: this goes to the ledge with the Green Key. Collect it, then go back to the long hall with the stairs you raised.
A stairway to the opposite end of the hall is now in place. Go up here, and activate either (or both) of the walls with the red and blue tapestries to open doors. Inside here is the Green Door. Through here is a tall hall with many large and fat columns, between which are stairways leading up and away. This is the Great Stair. After getting the enemies at this end cleared out, get your weapons ready: there's a Maulotaur waiting near the top. After defeating him, go to the top of the stairways, where you'll find two Blue Doors. Hit the switch on the wall nearby, then go back down to the long hall with the raised stairs, to the inset area where the switch is. You'll find the Blue Key in a now-opened nook in the wall. Collect it, but instead of going back to the Blue Doors, go to the overlook around the lava-feature room, to the far end. The secret area here is now open. There are items here of course, plus a teleporter that goes to near the top of the Great Stair, so you don't have to take the long way around again.
Back at the Great Stair, doors to compartments inside each of the fat columns will have opened. A bunch of enemies will have been released, but also there are a lot of items inside the columns, including a Map Scroll, armor and a weapon. When you've seen everything and had a fly-around if you want, go to and open either Blue Door. The exit portal is inside.