- "Few people have the honor of seeing Lord Vader in his sanctum. I suggest you keep all you witnessed to yourself."
- ―Vaneé, to Orson Krennic
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, as well as the ancient Corvax Fortress located underneath. 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 Darth 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 eight more designs, a perfect design was finally built with the ninth. 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 Maximilian Veers there about the over stretching efforts to defeat the Rebel Alliance.
- "You created this design, in your last body. Tell me what it is."
"The door to the dark side is locked. This is the key. Your beloved waits for you there, Vader. My fortress will tune the energies of the locus. It is your first step towards using the power of the dark side to pierce the veil of time, between life and death."
- ―Darth Vader and Momin
Fortress Vader was a castle designed by the Sith Lord Darth Momin and occupied by the Sith Lord Darth Vader. Momin intended for it to tune the Force energies from the dark side locus under it and open the door to the dark side of the Force. It was constructed by Vader himself as a testament to his own might, with the assistance of his Imperial resources. It was located on the site of his greatest defeat, and was built over an ancient Sith cave in the Gahenn Plains. The complex's location and design came as much from the dark side of the Force as it did Vader's personal history. The central tower was constructed out of obsidian, and its tuning towers served as conduits for the dark side. Within, Vader maintained a bacta tank where he would remove his armor and rest inside.
- "You have given me a wondrous opportunity, Lord Vader. I will not throw that away. This will be my masterpiece."
In 14 BBY, after a rogue Inquisitor chase, which Darth Vader successfully dealt with, Vader's master, Emperor Palpatine, rewarded him by offering Vader a chance to build a base on any planet of his choosing. Vader chose to live in the unforgiving environment of the volcanic planet of Mustafar, as it was highly linked to his connection with the dark side. Palpatine accepted and sent Vader to the planet, accompanied by the Imperial colonel Alva Brenne and Lieutenant Roggo, as well as the Mask of Darth Momin, which Palpatine had found in a hidden room in the Jedi Archives years prior and had already communicated with. Upon arrival, Vader scouted the area and decided to have the fortress to be built over an ancient Sith cave, which Vader had meditated in before. Vader returned to meditate in the cave, but was interrupted by Brenne, who had created a design for the fortress. Vader rejected it, and Brenne returned to the ship.
Upon her return, Momin's mask killed Colonel Brenne and possessed Lt. Roggo, finally giving Momin a corporeal form after many centuries. During his possession of Roggo, Momin created the first blueprints for the fortress, designing it in such a way to maximize its attunement to the Dark Side. Momin then showed these blueprints to Vader upon his return. Vader killed the possessed Roggo and took Momin's mask into the Sith cave, where the mask gave Vader a vision of Momin's past and his love for art, and then attempted to possess Vader, though Vader resisted. Shortly after, a group of Mustafarians attacked, and the mask possessed one of them. In his new body, Momin stated that the design he had for the fortress would act as a tuning fork for the Force, and promised Vader that if it was constructed, Vader would finally be able to achieve a way to meet his lost love, Padmé Amidala, again. Vader threatened Momin, stating that Palpatine had already lied about this to him prior and that Momin would be killed if he did not live up to his word.
- "Now, I can understand why you might have lost a bit of faith in me at this point. But I've said all along that this is a process. And my ninth design, well…I think it's really something special."
"One way or another, Momin…there will be no tenth."
- ―Momin and Vader
Construction began for Momin's design. With the aid of Vader's countless Imperial resources, the fortress was soon finished. Vader attempted to use it in opening the door to the dark side, but it failed and the fortress was wrecked. The pair retried with four more of Momin's designs, but they also failed. Every time Vader tried to open the door to the dark side it affected the rest of Mustafar, causing lava storms all over the planet to wreak havoc on the Mustafarians. Lava fleas began swarming the construction site regularly because of this. After the failure of Momin's fifth design, Vader killed the body his spirit possessed as a punishment and had his mask possess a stormtrooper. Following the failure of Momin's sixth design, Vader killed his newer body and had the helmet possess an Imperial officer. Momin had confidence in the seventh design, but it failed as well, and Vader replaced his body with a Lava flea's.
In 12 BBY, when Momin's eighth design failed, Vader once again killed his body and had the mask possess an Imperial construction worker. This time, Vader told Momin there would not be another chance. Momin had great faith in the ninth design. Vader began to channel the force from the Sith Cave and the pair saw it was working. However, Captain Junus, the commander of the fortress' garrison alerted Vader of another attack, one the Imperial forces could not defend against. Vader left the Sith cave to deal with this while Momin double crossed him, opening the door to the dark side himself and resurrecting his original body. As his mask greeted it, the possessed body of the Imperial construction worker handed over the mask and Momin was made one again.
- "There will be no evacuation. There will be no retreat."
- ―Darth Vader defends his fortress
The Imperial garrison attempted to flee until Vader intervened and turned the tide of the battle. From afar, the Mustafarian leader Kkkt and his fellow chieftains then used the Force to bring a flow of lava onto the Imperial forces. The battlefield was wiped clean, and only Vader survived. Kkkt brought his remaining forces to attack the fortress and Vader entered the Sith cave. From there, he channeled the Force through the tower and caused a lava storm to wipe out Kkkt and his army.
Momin appeared from behind and confronted Vader. Momin claimed that Vader was weak, that he ought to serve the dark side, rather than seek to control it, and that Vader would never be able to open the portal as the dark side would have rejected him. The two dueled for a time, while Momin lamented the apparent regression and dilution of the power of the Sith in the millennia since his death, calling them weak and "Jedi-obsessed." Vader, who had been damaged during the battle and had expended a considerable amount of energy destroying the Mustafarians, received several devastating wounds from Momin during their duel. Unrelenting, Vader reached out with the Force and crushed Momin beneath an enormous stone. Momin proclaimed that he would not die, for the dark side loved him, and Vader replied that if that was indeed the case, then he would be saved by the will of the dark side. However, Momin could not summon enough power to overcome Vader's, and Vader slammed the stone flat against the wall, killing the disbelieving Sith Lord. Vader proclaimed his destiny was his own, before opening the portal himself.
Vader's spirit left his body and entered the portal, in search of his beloved Padmé Amidala. During his search on the other side, Vader's spirit came across a vision of his fortress and its many former designs before it turned into the Jedi Temple. Vader's spirit soon returned, unsuccessful of his objective, and merged with his body again. After this, he used the fortress to once again tune the energies of the dark side locus and destroy the portal because of its failure to resurrect Amidala. After this, he contacted Palpatine, saying he was ready to return to Coruscant.
Not long after its completion, an Imperial Inspector named Vaneé was sent by Moff Seward to Mustafar with his droid, See-Seven, to determine why several of the garrison's stormtroopers had gone mad. He and See-Seven examined a trooper of the castle's garrison that had been exposed to the lava fumes and became a lunatic. During his questioning, the trooper attacked Vaneé and warned him not to go inside of the castle. He saw that the trooper was just weak-minded. Vaneé entered the castle, looked around and searched for the Sith Lord Darth Vader. He found Lord Vader in a bacta tank.
He told Lord Vader that the castle was not safe, should be evacuated and have environmental filters installed. Although he stated filters could be installed eventually, Vader refused for the time being, and Vaneé argued against it until the fumes affected him. He saw hallucinations and Vader offered him a position as his attendant. Vaneé heard the hallucinations telling him what Vader needed. Vaneé tried to convince him that the hallucinations were not real and tried to laugh at them. He laughed for days until his laughter became real. Vaneé agreed to become Lord Vader's attendant at the castle and told him his name.
Not long after Vaneé went into Vader's service, a large horde of Mustafarians launched a raid on the fortress. The Mustafarians, led by Criakan who rode upon a lava flea, entered through the fortress' hanger and quickly overtook the fortress' Lava Troopers. As they descended deeper into the fortress, the Mustafarians came upon Vaneé and quickly surrounded him. Vaneé started to speak of mercy when Criakan took him by his robes and asked why they should show him mercy when his master did not as he slaughtered their kin. However, Vaneé quickly pointed out that he was not speaking to them as Darth Vader appeared behind them with his lightsaber drawn. Vader proceeded to kill the Mustafarians.
Raid of Mother Ssl
Later Palpatine grew displeased with Vader, and so sent Rersey to antagonize Vader. Vaneé stood on the lower level of an audience chamber within Fortress Vader near Imperial advisor Rersey as his master stood on the upper level looking out the room's window with two Imperial Royal Guards standing below. Rersey was demanding an explanation from Vader which angered Vaneé due to the advisor's lack of respect towards his master. Vaneé reminded Rersey that he was a guest in his master's residence and would act accordingly which Rersey rebuffed. Vader halted the pair's squabbling as he felt a disturbance in the force moments before a chilling scream echoed in the chamber, startling all but Vader.
Vaneé along with Rersey followed Vader and the two guards as they went to investigate the source of the scream. Rersey remarked how the fortress was supposed to be secured to which Vaneé quietly suggested that Rersey secure his own mouth. When Rersey asked what he had said, Vaneé drearily replied he had said nothing right before loud heavy booming sounds echoed through the corridor. Arriving at the fortress' entrance, Rersey demanded to know what was going on to which Vaneé answered that the castle was under attack as a horde of plagued infested Mustafarians and a lava flea broke through the doors. Vaneé looked on in shock as his master and the guards engaged the rampaging horde which included an infected Lava Trooper. After Vader had been bitten by the infected trooper, Vaneé cried out in horror to Rersey that they had to escape only to look on in horror, clenching to Rersey as the horde and Vader slowly advanced on them until he was halted by Mother Sssl of Clan Rrrt.
As Rersey confronted Sssl, Vaneé cautiously advised him to be careful as she controlled the infected. After the advisor snidely asked Sssl what secrets she was protecting, Vaneé pointed out to Rersey that Sssl would not be telling them what secrets she was protecting. Vaneé and Rersey recoiled in fear as Sssl commanded Vader to kill the both of them as Vaneé cried out in fear. Rersey then shoved Vaneé to the floor between him and Vader as he told the Dark Lord he needed to resist Sssl's power over him. As Vader advanced towards them, Vaneé cowered in fear on the floor until the Dark Lord revealed he was not under Sssl's power by levitating her off the ground.
Getting up off the ground, Vaneé revealed to Sssl that it had all been a trap to lure her into attacking the castle. As Sssl struggled in Vader's grip, Vaneé gleefully told her that she was no match for his master's ingenuity and power. Vaneé watched as Vader willed Sssl to order her horde to drag her into the lava and followed his master and Rersey outside as they watched Sssl and her infected horde burn in the lava. Once the ordeal was over with Vaneé congratulated his master, stating how all Mustafarians would tremble at the very mention of his name. Vader informed Vaneé that it was of no importance before telling Advisor Rersey that his shuttle was waiting for him to take him back to the Emperor. Vaneé looked at Rersey with an expression of exasperation as the traumatized advisor continually recounted the sight of Mother Sssl being dragged into the lava.
In 5 BBY, the Jedi fugitive Kanan Jarrus had heard rumors about Mustafar, specifically referring to it as a place where Jedi survivors were sent to die if captured by the Galactic Empire. Before Jarrus could be taken to the surface, however, he was rescued from the Star Destroyer Sovereign in orbit by his fellow rebels.
Around 1 BBY, members of the Rebel Alliance conducted a mission to infiltrate an Imperial base near the castle for Captain Cassian Andor. Disguised in stormtrooper armor the four traveled to Mustafar with their pilot Athex and the reprogrammed security droid K-2SO in a stolen Imperial transport. Upon arriving, the Imperial base was not happy to hear of the ship's arrival and demanded the ship land immediately and be ready for inspection. A hover platform docked with the ship and took the spies across to the main base, before taking a lift down to the base. They found a rack of blasters inside and the alarms started going off. K-2SO informed the others of this and instructed them to pick up the blasters and find their target. As the base responded to the threat, stormtroopers filed out onto walkways to try and shoot the rebels. They were ultimately defeated, but then insectoids rose from the lava and spat fire at the infiltrators. A giant insectoid backed them up until the spies killed it as well.
K-2SO unlocked the door for the rebels to enter inside the base. As the rebels moved back into a corridor, K-2SO presented them with a memory puzzle whilst Stormtroopers were shooting the rebels from behind. They then ran down another corridor leading to a room containing the weapons crate the rebels were dispatched to get. Athex opened the crate and found a proto-saber. However, Darth Vader arrived and threw his lightsaber at them, stabbing Athex in the chest. The infiltrators fired at Vader and the Stormtroopers that arrived to reinforce him. Vader used the Force to crush their weapons and began walking towards the party. K-2SO then rammed the Imperial transport into the side of the building, scooping up the spies and the crate. After leaving the atmosphere, they jumped to hyperspace.
In 0 BBY, Vader was meditating inside the bacta tank in his sanctum when Director Orson Krennic arrived. Sending Vader's assistant Vaneé to retrieve him, Krennic and Vader discussed the Death Star. Krennic wanted more credit for the project but Vader refused to help him. When he asked if he still had command, Krennic was Force choked by the Sith Lord as a warning. After the meeting, Vaneé told Krennic to keep all that he saw of the castle to himself.
- "We've no ship, barely any weapons, and the only thing on the planet is a huge spooky fortress."
- ―Hudd, upon seeing the fortress
At some point, the Auric, a Rebel YT-1760 Small Transport serving under Commander Lina Graf and her crew, was shot down by TIEs above Mustafar and crash-landed near the castle. After barely escaping the ship as it sunk into the lava, Graf decided to go to the fortress in order to find a way off the planet, much to the protest of Lieutenant Thom Hudd. After removing their Rebel insignias and rank pins, she and her crew traveled to the castle, but were confronted by the Castle's Lava Troopers, which shot at them.
Graf and her crew were able to escape the Lava Troopers while XM-G3 kept them busy and reached an entrance into the castle. Hudd sliced the door open and they walked inside. CR-8R questioned the structure while the technician Skritt feared there was a trap waiting for them. Graf knew of a base on the other side of the fortress and assured them they would be getting out. To calm Skritt's nerves, she told him a story about the Emerald Witch. G3 returned to them, having defeated the Lava Troopers, and Hudd decided to try and steal something and found an artifact in one of the rooms.
Suddenly the rest of the crew heard him scream and ran to where he had gone, only to find no trace of him. They split up in search of Hudd and G3 went off with Skritt, telling him a story about the Gorax along the way. They found the castle's attendant, Vaneé, in the control room. Vaneé was reporting their presence to Vader when G3 confronted him. After telling Vader they had found him, he watched as the Dark Lord of the Sith arrived and ripped G3 apart in front of Skritt. Skritt reported this to Graf, and she left 8R, only for him to be attacked by Vader straight after.
Graf and Skritt ran down a hallway where they came under attack by security droids under Vaneé and Lord Vader. Graf tried to shoot Vader, but he was able to withstand and deflect her shots. After Vader tossed Crater at Graf, she and Skritt made their escape to the castle's hangar. There, a Kubaza beetle attacked the Lava Troopers, while Graff and Skritt were able to steal a TIE fighter and escape. However, Hudd got captured.
Prisoner of Fortress Vader
Not long after that, Hudd was awakened by Vaneé, and realized that he was hanging on a torture web while Vaneé told him the story of a "Horned Devil" After finishing the story, Vaneé commented on the nature of greed as he deactivated the hologram that showed the story, and he concluded how it never ended well before pointing an accusing figure at the weakened rebel. Vaneé continued to electrocute Hudd with the torture web. After enjoying some time tormenting Hudd, the servant of Vader thanked Thom Hudd that for being able to inform Vader that the torture web was still operational before electrocuting Hudd again. Hudd then told a story of the former imperial officer Cremp and of a failed experiment that turned into a monster. After finishing the story, Hudd escaped the web without Vaneé's notice, and after realizing that his prisoner was gone, Vaneé pursued him, telling him that he would experience new levels of pain.
Vaneé chased Hudd down the room and found him hiding behind a red curtain. Hudd managed to escape just as Vaneé was about to cut him down. As Hudd ran and the curtain tumbled down on Vader's servant, he told him that there was a time for everything. Vaneé reassured Hudd that they couldn't change their nature and went on to tell of the story of Asajj Ventress and her encounter with a horde of baby sarlaccs during the Clone Wars. After finishing the story, Vaneé concluded that the dark side always won as Hudd found a room in which he believed contained the map of the castle. Vaneé managed to summon a group of Imperial training droids to pursue him. The droids began to attack Hudd, but Vaneé halted the droids to not hurt him. As the droids began taking Hudd back to the Torture Web, Hudd began to tell the story of Jabba's cousin Crakka and the B'omarr monks. After finishing the story, Vaneé told Hudd that he wanted to hear real screams as he prepared to electrocute Hudd again, only to be stopped by an Inquisitor known as the Fourth Sister, who said to the sadistic servant that it is by orders of the Emperor that Hudd is to be taken to Coruscant. Vaneé said to the Inquisitor that she has no authority whatsoever in the castle as the two began to duel.
Vaneé went on to explain that she sounded just like Imperial Adviser Rersey when he landed on Mustafar years ago. After finishing the story, Vaneé explained to the Fourth Sister that being a lackey of the Emperor wasn't enough, but that they must earn their respect within the fortress, just before electrocuting the Inquisitor to reveal that it was Lina Graf, who has come to recue Hudd. While Graf was fighting Vaneé, Hudd managed to reactivate the distorted holograms at Vaneé, causing him to not see for a moment just as Graf and Hudd ran to the ship and escape into hyperspace. Vaneé noticed that Hudd escaped, and said to himself that Vader must not know of this just as Vader appeared before him. Vaneé tried to explain what happened, but Vader had him electrocuted in the Torture Web before imprisoning him in a deep pit below the castle.
Weeds in a storm
Following the War on Shu-Torun, Vader met with Admiral Kendal Ozzel and General Maximilian Veers to discuss the Empire's construction projects that had been delayed due to the Death Star's destruction. Ozzel and Veers stood behind Vader as the Dark Lord looked out the window of the chamber. Vader spent the majority of the meeting looking out over the Mustafar landscape. He asked for Ozzel to report on the growing rebel nuisance in the galaxy. Ozzel tried to explain the difficulty in destroying them and suggested mobilizing Vader's new Death Squadron. Veers then claimed the difficulty in destroying them was the fact they were always spread out, suggesting that catching them together may prove more of a blow. Vader acknowledged this and devised a plan to hit a blow to the rebellion.
Fall of the Fortress
Following the death of Vader, the residents of the castle departed, leaving an imprisoned Vaneé behind. Not long after, two young Mustafarian boys named Tuttel and Giggek attempted to blow up the Fortress to symbolize the demise of Vader. Tuttel's uncle Lellis tried to stop the boys from going there, warning of the evil that may be there, however Tuttel and Giggek ignored the old Mustafarian and made their way to Fortress Vader. Upon reaching the castle, the two young Mustafarian boys planned to collapse the fortress by blowing up the bottom. Tuttel became scared and wondered if Lellis was right. Giggek teased Tuttel by telling him of the story of the Sorcerer's Jewel, in which Tuttel said he wouldn't believe in such a thing. After the conversation, the two Mustafarians heard the voice of seemingly helpless old man (who was actually Vaneé). The boys thought it was an illusion set up from the lava fumes, but came to accept that the individual standing before them is a real person. The servant said that Vader was angry with him, and was the one responsible for his imprisonment. Giggek replied by saying that Vader was gone, in which Vaneé expressed surprise upon hearing the information. He explained of a time when an Imperial scientist named Rersey attempted to kill Vader and Vader thwarting his plan.
After explaining the event, Giggek told Vaneé to put his hand up so that he may help him get out. Vaneé pulled Giggek down, and a shocked Tuttel asked who he was. The servant explained that he was "The one true Acolyte of the Beyond", and revealed his name and explained that he was death to all who profane the name of the Sith. Giggek was still alive, and told Tuttel to run as fast as he can and tell everybody about they did. Giggek detonated the bombs, and the bottom of Fortress Vader exploded; however, the castle still stood. Giggek's sacrifice was not in vain: Vaneé somehow managed to survive, and as he was limping away from the ruins a voice came to him. The Voice told Vaneé that he knew him, the one who spoke to him at the bottom of the pit. Vaneé knew that this voice was Vader's, and asked him of what he wants him to do. The apparent ghost of Vader told his servant that it was not over, and that he will rise out of the lava river and be made whole and become more powerful than ever before.
Haunting of the Ghost
- "I call upon the Immortal Gods of the Sith. On malevolence of anger, and dread. Reach across the stars, spirits of the Obsidian Flame. Seek out those who have wronged us. For the Dark Lord will rise again. The Dark Lord will have his revenge…"
- ―Vaneé, in his ritual to remake Vader
In 5 ABY, Vaneé told the apparent ghost of Vader that everyone in the galaxy believed that Vader had died, to which Vader's ghost replied that he will resurrect and become more powerful than ever, as he explained it was the will of the dark side. Vaneé called upon the Immortal gods of the Sith and quoted that the Dark Lord will have his revenge. After the ritual ended, a nightmare came on Milo Graf, the younger brother of Lina Graf, about a dream in which sometime during the Clone Wars, C-3PO, R2-D2, and Jar Jar Binks got infected with a mysterious disease. After telling Crater of the dream he had, he assured him that he would never do such a thing, before Crater, who somehow was possessed by Vaneé, stunned him, telling him that he would never hurt anyone unless his Dark Lord commands, before taking a passed out Milo to Mustafar.
- "In the castle of your kin there is a domain of darkness. There you will find an ancient wayfinder. That wayfinder will reveal a path to one you seek. Be warned—secrets weary of their tyranny."
- ―Eye of Webbish Bog, to Kylo Ren
The fortress was left in ruins and vacant after the Empire's end. Its grounds eventually came to be protected by the Alazmec of Winsit, a Sith cult who colonized Mustafar and protected Corvax Fen.
In 35 ABY, the Alazmec colonists guarded the Sith wayfinder at the fortress ruins in Corvax Fen. The colonists were ultimately cut down by Supreme Leader Kylo Ren of the First Order and his stormtroopers, who had traveled to Mustafar to collect Vader's wayfinder that led to Exegol.
Behind the scenes
- "Ralph McQuarrie actually drew quite a few small thumbnail sketches [of Vader's castle] that were very intriguing. They were kind of angular versions of a tower, and I saw the potential of where he was going, and I just exaggerated that quite a bit."
- ―Doug Chiang
Fortress Vader first appeared in the Star Wars Anthology film Rogue One: A Star Wars Story, which was released in 2016. Early story meetings between Leigh Brackett and George Lucas for Star Wars: Episode V The Empire Strikes Back had them discussing Darth Vader having a black castle surrounded by lava, gargoyles and gremlins. Inspired by the unused Ralph McQuarrie sketches for Episode V, the design team for Rogue One gave the castle a tuning fork shape, leading director Gareth Edwards to suggest that it is "tuning the dark side". The concept was also used for the Temple of the Kyber, which represents a mirror image on the light side.
During the concept phase of the 2015 film Star Wars: Episode VII The Force Awakens, concept art titled "Vader Castle" was created by Erik Tiemens and Brett Northcutt, with the piece depicting TIE/IN interceptors exiting a fortress near a river of lava. In Star Wars Legends, Darth Vader had similar abodes, including a palace on Coruscant and the more well-known Bast Castle on Vjun.
In the Rebels Recon behind-the-scenes video for "Ghosts of Geonosis," Pablo Hidalgo directly confirmed that Darth Vader's castle is specifically where Tarkin was taking the captured Kanan Jarrus in the Season One finale of Star Wars Rebels. At the time, the series only said that he was being taken to Mustafar, and that Kanan had heard it was "where Jedi go to die." Even in Season One, Hidalgo knew that they were taking him to a castle Vader has on the surface - this wasn't established retroactively but was planned out. Hidalgo also directly established that Vader's castle was definitely constructed on Mustafar by the start of the series, but he stated that he wouldn't reveal exactly when and how it was built.
A hologram of the early design for Vader's castle can be seen in the twenty-first issue of the Marvel comic book series Star Wars: Darth Vader (2017). This was one of the early designs by concept artist Erik Tiemens during the concept phase of Rogue One, supplied by Lucasfilm Ltd.'s art department as noted by Phil Szostak.
Fortress Vader appeared in the non-canon LEGO Star Wars Terrifying Tales, which premiered on October 1, 2021. Taking place in 35 ABY, Fortress Vader was made into a luxury hotel by Graballa the Hutt. After becoming stranded on Mustafar, Poe Dameron and BB-8 are confronted by Vader's servant Vaneé, who recites scary stories as he leads them into the depths of the castle.