After entering the blue key door, head down the stairway and a wall will lower in the west. Head through the opening to find a berserk pack. (sector 144)
At the south of the level, if you drop off the south or southeast sides of the central, metal platform, an arrow will appear pointing at the east side of the red column. Either before or after you do this, you can open this column to reveal a teleporter, leading to a megasphere. (sector 295)
Once you return to the northwest area after obtaining the yellow skull key, arch-viles will have teleported in. In the northeast is an alcove, with a chaingunner inside, that cannot be reached by normal means. To get there, after the chaingunner is killed, you must have the nearby arch-vile initiate its fire attack against you, and run toward this alcove when the blast goes off. If done correctly, you will be thrown into the air, and into this alcove, where you can obtain combat armor, a backpack, a computer map, and an invincibility. (sector 363) The technique used to reach this secret is known as an arch-vile jump.
It is possible to get trapped in the area with pitch black walls if played on Ultra-Violence or higher. After teleporting to the area the player is standing on a teleporter pad (sector 334), facing a cacodemon and a shooting target. If hitscan weapons are used (pistol, shotgun or chaingun are available up to this point), two walls (sectors 338 and 341) will open, enabling further progress. Nevertheless, if the player uses the plasma rifle against the cacodemon and then makes a step forward, he will fall into an inescapable, 25-unit high pit. From there, the only way to open the walls is to shoot the target with the super shotgun, but this weapon is missing in the area on skill levels 4 and 5.