Fortress Vader, also known as Darth Vader's castle and Vader's personal abode, was a stronghold located on the volcanic world of Mustafar. Once the site of a Sith temple, the fortress was built above a Sith cave that still remained. Existing by the time of reign of the Galactic Empire, the Sith Cave contained a dark side locus. The castle was designed by the preserved consciousness of the ancient Sith Lord and artist Momin, whose haunted mask possessed an Imperial lieutenant and made him create the fortress's blueprints. Momin meant for it to channel the Force energies from the dark side locus under the castle and open the door to the dark side. The Sith Lord Darth Vader allowed Momin's design to be built using Imperial resources in his possession. Although Vader failed to open the door with the first design, after a trial and error process involving seven more designs, a perfect design was finally built. It was able to open the door to the dark side, but when Vader was distracted by a Mustafarian uprising, Momin opened it for himself and resurrected his original body. Despite Vader's intervention, the army of Mustafarians were able to wipe out the Imperial garrison. Vader retreated to the castle and used its power to do the same to the Mustafarians. Momin then confronted Vader and was able to badly damage Vader in their brief duel. Vader crushed him using a stone slab. With Momin, the Mustafarians, and the Imperial garrison dead, Vader was left alone to open the door to the dark side himself. Vader's spirit entered the portal in search of his dead wife, Padmé Amidala. He returned unsuccessful and used the fortress' power to destroy the portal. He then contacted his master, Emperor Palpatine, to confirm his planned return to Coruscant. The castle came to serve as Vader's personal residence and from time to time, he rested inside a bacta tank. On one occasion, his stasis was cut short by Director Orson Krennic of the Death Star project. The two discussed the project at hand. Vader also conversed with Admiral Kendal Ozzel and General Maximillian Veers there about the over stretching efforts to defeat the Rebel Alliance.