The Fortress of Doom was the name of Doctor Chaotica's base of operations, located on Planet X. It housed Chaotica's throne room and his death ray, which he used in his attempts to conquer Earth. It also contained a detention area called the Dungeon of Pain and the Cradle of Persuasion.
The Fortress of Doom was part of Tom Paris' The Adventures of Captain Proton holo-programs. Accessible by a drawbridge and underground caverns, the fortress contained many of Chaotica's evil inventions and henchmen, as well as his harem. It was protected by a lightning shield, which made infiltration by even Captain Proton quite difficult.
The lightning shield failed to protect the fortress, allowing it to take heavy damage when the appearance of photonic lifeforms caused the program to be left on continuously, setting off a war between them and Chaotica. (VOY: "Bride of Chaotica!")