This level is the only one in the game in which you can see that you may have walked here rather than just teleported through a magic portal. Look behind you where you begin the level, and you'll see a stairway going up and around a corner, blocked by a locked gate. It's a nice touch, and should have been done more often in the game, because it gives a better sense of continuity even though you do exit levels via portals.
You'll be in a smallish hall, with interesting areas on either side that are completely surrounded by water, even overhead. The windows into the areas will convert into doorways later on, allowing you to get to the items. You'll be under attack by Disciples of D'Sparil from both areas. A tip here is to use a powerful weapon such as the Phoenix Rod to quickly clear out one side, then you can take care of the other side in relative safety. Go to that small area at the far end of the room where the weapon and ammo are. A door to the area—a lift—will close up when you enter it. Hit the switch, and the lift slowly lowers. The lift will pass by an alcove in the wall, and continue lowering. Step into this alcove: there's an item here. If you miss it, you can fly back into here later.
At the bottom of the lift, go through the door and into the next room. You'll find the Yellow Door to your right, the Green Door to the left, another door ahead, and a teleporter. The teleporter goes back to the first room; if you go here, the watery side rooms will be open. Down in the lower room, go through the far door that doesn't need a key. In here is a long elaborate hall with a big pool in a sunken area taking up most of the room.
At the bottom of the lift, go through the door and into the next room. You'll find the Yellow Door to your right, the Green Door to the left, another door ahead, and a teleporter. The teleporter goes back to the first room; if you go here, the watery side rooms will be open. Down in the lower room, go through the far door that doesn't need a key. In here is a long elaborate hall with big pool in a sunken area taking up most of the room. There's a fast wind or current down in the pool. On one side you'll find two more doors plus a door with the exit symbol. First, take a look at the far end of the hall for a stained-glass window, and a stairway in front of it going down into the pool. When you step onto the stairs from the pool or vice-versa, the stained-glass panel will open, revealing a shallow compartment with items inside. Down in the pool, there's a platform at the end opposite the stairs with an Ethereal Crossbow. If you step onto this, go back to the main door to this hall: the wall across from here has opened (the wall with the blue/gold tapestries), and there are items and some enemies inside.
Take either of the side doors into the next room over. First, clear out the enemies in those three dark cages, so they won't be pestering you later. At one end of the room there's a window too low to get through, looking onto a wide curving passage, one end of which leads down into darkness. Two more doors lead beyond this room. Go through the right door first, and clear out the enemies. Activate the wall between the two torches: there's a hidden area with items here. At the end of this passage is a large window looking out into the long hall the left-hand door outside goes to. This window is hidden from the other side, and now the enemies that were in here won't be firing at you later.
Jump down into the hall (or go around and through the door) and continue on. Watch out for the low ceiling between the four pillars: this is a crusher that will activate when you step under it. But one very nice thing about this crusher is that you can use it to squash the enemies roaming around here, by getting them to follow you under it. There are three ways to go from where the crusher is. To the right is an area with ammo and a Mystic Urn in the nook at the end. Ahead is a stone column with a compartment on the opposite side. Quickly turn around if you step into here: you'll get a good-size Undead-Warrior ambush, but also some additional items. Take the left-hand route from the crusher. Just before the door, activate the left-side green column that has the gold symbol: there's a secret area inside here. The window to the right is another of those that's hidden from the outside. In here, in addition to the enemies you can clear out, are items and ammo. Go back out and through the door to the area past the window, and collect the Yellow Key in the next room up the step. (That section of floor that's slightly darker suggests that it's a lift. It is in fact, but it activates from the other side.)
Two spiral passages
Go back to and unlock the Yellow Door. In here is a spiral passage going down and off to the right, getting slightly darker as it does. At the end of this is the Green Key and ammo. Pick it up and go back to the Green Door. When you got the key, a wall here opened, connecting to the spiral passage you saw through the low window earlier. At the end of this passage, in the dark, is a platform with a Hellstaff. Activate the marked texture behind it for a hidden area with items. Also, when you open this door, another hidden door momentarily opens. Stand where the Hellstaff was, open this hidden door, then turn around and look straight ahead of you, along the outer wall of the spiral passage. (If you still don't see it, look on the automap or activate a Torch.) This second door doesn't open from the inside; to leave, take the teleporter and follow this route until you get to a room with a gold symbol on the wall. Activate the wall with the symbol to open a door leading back into the long hall with the pool.
Go to and open the Green Door. In here you 'll see two long cages full of monsters. Past here you'll find a long window looking out into another of those watery areas like the ones near where you began the level. This one won't be accessible, so you won't be able to get to any items the Iron Liches drop. There are two ways to go from here. Go left, and follow this route, past two side rooms with enemies, to the end to where the Phoenix Rod is. When you step here, a side room back the way you came has opened. It's a bit tough to see, because it looks like it was always there; follow the right-hand wall past the window to the side room and to the wide section of the hall. Go through the narrow entrance and hit the switch inside the room inside. This opens the door in the hall-with-pool with the exit symbol. If you're aiming to kill every enemy, you can go there now, where there's a solitary Disciple of D'Sparil and an exit portal to E3M5: The Ophidian Lair. But don't exit here: there are better things in store. Also, when you went up to the Phoenix Rod, those two long cages with the monsters have opened, and you can get to the items inside.
Go to where the large watery area and monster cages are, and this time take the right-hand way, which goes down a long stairway and off to the left. Past here is a long hallway with flashing light and several nooks on either side with pits in them. At the end is a lift: the one where the Yellow Key was. Drop down into nearest nook on the right (actually, any of them will work, but most contain enemies). The floor to this and all of the other nooks will rise to the level of the hall. This releases the enemies, but also there's a great deal of ammo and items in the nooks, including a Map Scroll. Last, go back to where the Phoenix Rod was. There are two hidden doors, next to the torches, that go to the two rooms inside the windows. These will be filled out on the automap, now that you have the Map Scroll. At this point, you'll have seen everything—almost. Don't take the level exit, because...
Secret Level Alert
This episode's secret level is accessed from this level. Go to where the Phoenix Rod was. When you hit the switch that opened the exit room, another door here opened. In here is a second exit portal, which goes to the secret level, The Aquifer.