Heretic/E1M5: The Citadel

Yellow key
After starting on the southern edge of the map, The yellow key is located on the left building, and can be accessed by climbing the stairs on the left-hand side. There will be a large number of golems on the stairs, trying to block access. At the top of the stairs is a point where you can jump to the southern building section, allowing you to reach the yellow key.

Green key
The green key is in the eastern building, and can only be accessed once you get the yellow key.

A door leads to the southern part of the eastern building, which leads to a small staircase. The stairs lead to a green key, and also allow a running jump to a tome of power.

You also need to flip a switch found in the northern section of the eastern building. In addition, if you climb the stairs in the northern part of the building, you can reach the etheral crossbow at the end of the long corridor.

Blue key
With the green key, you can head to the south-east or south west to go through the green key gate. From here, you can access the northern section by running down a long corridor. Some of the wall textures to the center may look different, and they can be opened to reveal a new path.

In the northern section, head to the center river and move to the southern part. This contains the blue key, allowing you to reach the exit.

Exiting the map
The door will not open if you try using it directly. To the left is a switch that opens the door, allowing you to escape.

If desired, you can jump into the center pillar to collect wings of wrath. This allows you to fly and collect any other secrets on the map.

Secrets

 * 1) In the south-west building, the northern staircase leads to a door. next to this door is a secret door containning ammo.
 * 2) In the northern section, the western wing has a ground-level doorway on the north-west corner. THis contains a switch which may be used; quickly run out, take a right-hand turn, and look for a lowered lift.  Collect the Morph Ovum before the lift heads back up.  (This also provides an alternate path for the Dragon Claw.)
 * 3) After collecting Wings of Wrath, head to the south-east section of the northern section.  Fly into the building to collect a ring of invulnerability.
 * 4) After collecting Wings of Wrath, fly onto the platform just south of the blue key.  The platform leads to two teleports, each of which lead to a tome of power.  If you follow the flowing river, there is also a second Wings of Wrath in the center area, which is meant to be used if you get stuck.

While not classified as a secret, there is a shadow sphere in the south-east building.