Heretic/E3M6: The Halls of Fear

Yellow key
Enter the hallway to the north, but drop down to the water below. Look for a cave on the south of the eastern wall, and reach the yellow key at the end of the tunnel.

When you reach the yellow key, the wall ahead of you will lower to reveal an Iron Lich. You can evade it or fight it at your preference, although there is a dragon claw at the back of the revealed room.

Return to the starting area - the easiest route is through the teleport at the north end of the water.

Green key
From the starting area, run north, and turn west through they yellow key gate. This opens to a large corridor filled with weredragons and saberclaws hiding in the alcove - they will only approach if they can see you. When you are ready to cross the corridor, head to the left, and press the south switch to open the west and east doors (as well as the sub-corridors to the north and south). The north switch adjusts the starting area, making it easier to reach a secret below.

Enter the west corridor, and proceed to the southern end. Find the switch on the north wall and press it. Return to the north end of the corridor, and press the revealed switch in the north west corner. Return to the starting corridor, and the green key should be to the north and reachable. (If the door back is closed, you can press the switch to the south, or use the teleporter in the middle of the west corridor.)

Blue key

 * Caution: A maulator is somewhere behind green key door. Make sure you have room to retreat in case he is at the door.

In the middle of the west corridor, open the green key door on the right. You first need to unblock the stair by pressing the switch at the north of the watery area. Next, climb the stairs to the south to retrieve the blue key. When you reach the blue key, the water level will lower - if you fall in, you can escape using the teleporter behind the iron liches.

You can return to the starting area by heading east from the blue key. Turning north (as opposed to jumping back into the water) will open the rest of the map, so that you can explore.

Map exit
The exit is behind the blue key door, at the south of the western corridor. Follow the blue-colored rooms.

Note that entering here will create another pathway to the main level, which leads to an area tagged as a secret.

Secrets

 * 1) When you have entered the yellow door, you will find yourself in a gloomy hallway with many alcoves. At the western end, there are two switches. You need to press the southern one to open the other main door but the switch will also open the back walls of some of the alcoves. Enter the exposed corridor to the north to get a Silver Shield along with health and ammunition pickups.
 * 2) Similarly, a corridor to the south has been revealed. The place holds a Mystic Urn, a varying amount of Quartz Flasks and some ammunition.
 * 3) The northern switch will raise the starting area so that you can step on its fence and run over a gap to a new zone. A Phoenix Rod and an Enchanted Shield can be found up there. North of the weapon is a passage that will take you back to the start - with a Tome of Power - but a door will open to the west while you proceed. It will reveal a shortcut staircase to secret 2, but when you enter the familiar area again and face east, you will notice that an alcove with a Bag of Holding has become accessible. The place counts as a secret.
 * 4) When you have the bag, return to the bottom of the staircase you came from and go where the Phoenix Rod was. The doorways around the weapon can now be entered to reach a Map Scroll.
 * 5) Another staircase connects to the previous secret area. When you are the bottom step, face north and press the wall to lower it as an elevator. Above, there is a room with many timebombs, a new Bag of Holding and some nitrogolems as well.
 * 6) Go to the end of the aforementioned staircase and press the back wall to open it. Another door will open right behind it revealing a varying amount of ophidians, a Pile of Mace Spheres, a second Mystic Urn and yet a third Bag of Holding.
 * 7) At the blue door, you will find two banners. Go through the false wall between them to find a room with two ophidians that were placed there to fire through a false window. In addition, the area contains a Morph Ovum, an Inferno Orb and Greater Runes.
 * 8) Just when you enter the water of the maulotaur yard behind the green door, two wall pillars in the northern staircase back on the other side of the door will open. They will reveal a chamber with a Shadowsphere.
 * 9) Symmetrically with the previous case, another chamber will open at the southern staircase. It holds a Tome of Power. Note that both secret areas also have disciples and Mace Spheres in them.
 * 10) When you are about to take the blue key, the water level of the maulotaur yard will go down and a sheltered area will be exposed on the bottom. There are the Wings of Wrath, ammunition for the Hellstaff and a varying amount of iron liches. Use the wings or the teleporter to return up.

Although non-official, the water area that surrounds the starting position has a secret door at the northern end of its western side. The door hides a teleporter and a Ring of Invincibility. Use the teleporter to reach the ledge on the eastern side. Collect the Hellstaff and the ammunition. (You will have to use this path if you fall into an enclosed area after the nearby bridge raises.)