During the Imperial Era, to ensure the safety of Fortress Vader, a garrison was stationed on Mustafar. This garrison, commanded by Captain Junus during the construction of the fortress, met its end in the Battle of Fortress Vader. Subsequently, the garrison was expanded to battalion strength, maintaining security for both Fortress Vader and a nearby Imperial base.
Initially, the garrison tasked with safeguarding the construction of Fortress Vader in the Gahenn Plains of Mustafar was composed of extensive resources from the Galactic Empire. While Imperial construction teams worked on the fortress's various designs, magma troopers provided defense. Imperial officers were also part of the garrison, which deployed AT-ACTs, AT-STs, and TIE fighters for enhanced vehicle support. At one point during the construction, an Imperial-class Star Destroyer was stationed in Mustafar's atmosphere above the fortress. Captain Junus served as the commander of the garrison during this construction phase.
Following its destruction in the Battle of Fortress Vader, the garrison was rebuilt to continue its security duties at the fortress. Positioned on the far side of the plains, the fortress's security forces included lava troopers, security droids, and Imperial Royal Guards. AT-DPs were added to the security's arsenal.
Around the time of the mission to Mustafar between 2 BBY and 0 BBY, an Imperial base with additional security was established near the castle on the lava lake, supplemented by standard stormtroopers. Admiral Gable Karius commanded the Imperial garrison during this period, which was considered a battalion. Furthermore, Karius had an Interdictor-class Star Destroyer under his command, and standard stormtroopers provided security for Fortress Vader itself. Imperial forces were also occasionally present at Lady Corvax's sanctum, located beneath Fortress Vader.
During the Imperial Era, Darth Vader journeyed to Mustafar with the intention of establishing his personal stronghold. Following the activation and revelation of the Mask of Lord Momin to Vader, construction commenced on the castle. A garrison was formed to protect Fortress Vader and the construction teams during this period. The garrison defended the construction site from lava fleas, whose attacks intensified with each activated and subsequently destroyed design of the fortress. It was only with the ninth design of Fortress Vader that the structure maintained its integrity upon activating the locus of the Force found in the Sith cave beneath the fortress. While Vader activated the fortress, the garrison and castle were attacked by a Mustafarian army led by Father Kkkt, which resulted in the Battle of Fortress Vader.
Despite Captain Junus's recommendation to evacuate the garrison, Vader joined the battle and countermanded his orders, leading the garrison against the attackers while Sith Lord Darth Momin was resurrected from the past by his mask upon the fortress's activation. The entire garrison's ground forces were annihilated by a wave of lava created by Father Kkkt, though Vader retaliated by doing the same to the Mustafarians after re-entering his fortress and reactivating it.
Following the Battle of Fortress Vader, the garrison was reformed to continue securing the fortress. Shortly after construction was completed, the castle's security forces were involved in a raid on the castle, where a group of Southern Mustafarians led by Criakan managed to infiltrate the castle, only to be defeated by Vader. The fortress continued to be targeted by Mustafarian raids during its existence.
Later, Mustafarians again attacked Fortress Vader, this time led by Mother Sssl and wielding a mind-controlling plague. Mother Sssl managed to infect patrolling lava troopers, causing them to succumb to the plague. However, Darth Vader once again defeated the Mustafarians, slaughtering them all.