MAP14: Fortress of Evil is the fourteenth level of Icarus: Alien Vanguard. In this level, the marine must infiltrate a castle on another planet where demons reside. It was designed by Robin Patenall, and uses the music track "Hell" by Tom Mustaine.
In the prison where you find the blue skull key, a section of the western wall in the southwestern cell block is covered in moss. Open it to find a teleporter leading to the arachnotron cage (sector 158) in the northwestern battlement where you can get a bulk cell, a box of rockets and a computer map.
The portion of the wall south of the kitchen's southern door is covered in grime. Open it to find a teleporter that takes you to the arachnotron cage (sector 368) in the southeastern battlement where you can get a box of bullets and a box of shotgun shells.
In the eastern battlement, a section of the western wall between the blue and the red door is covered with grime. Open it to find a teleporter to the arachnotron cage (sector 97) in the northeastern battlement where you can get a box of bullets and a box of rockets.
In the western storage facility, the back wall of the northwestern corner is covered in grime. Open it to find a teleporter leading to the arachnotron cage (sector 364) in the southwestern battlement where you can obtain a bulk cell and a box of shells.
When you grab the red keycard, the two southern buildings open to reveal one (on easy skills) or two (on medium skills) spider masterminds or a cyberdemon (on hard skills). Leave the castle and look in the westernmost of these buildings to find that half of the floor and ceiling are covered in moss. Cross the divide between both halves, and then reenter the castle. Backtrack to where you found the yellow skull key: to the north of the table (sector 372) with 16 armor bonuses, 14 health bonuses and a megasphere, ascend the eastern stairs and follow the walkway to its end to find that the western wall has opened, allowing you to jump onto that table to acquire the aforementioned items.