The Fortress of Doom was the stronghold of the villain Doctor Chaotica on Planet X in Tom Paris's black-and-white holodeck serial The Adventures of Captain Proton.
The fortress is a loving parody of 1930s film serials, with sparking machinery, a throne room, dungeons and a death ray. Paris plays the hero Captain Proton and Harry Kim his sidekick Buster Kincaid, as first shown in Night.
It takes centre stage in Bride of Chaotica!, when photonic beings from another dimension enter the programme and take its ruler for a real tyrant. To end the war that follows, Janeway plays Arachnia, Queen of the Spider People, and enters the fortress as Chaotica's intended bride to lower its lightning shield so that Proton can destroy the death ray.
The setting returns in Shattered, when Chakotay and a past version of Janeway cross a time-fractured holodeck.