The Fortress of Doom was the name of Doctor Chaotica's base of operations, located on Planet X. It housed his death ray, which he used in his attempts to conquer Earth. It also contained a detention area called the Dungeon of Pain.
The Fortress of Doom was part of Tom Paris' The Adventures of Captain Proton holo-programs. Accessible by a drawbridge and underground caverns, the fortess contained many of Chaotica's evil inventions and henchmen. 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!")