Infinity Castle

The Dimensional Infinity Fortress was the lair of the Twelve Demon Moons and the primary location of the Dimensional Infinity Fortress Arc.

History
The Dimensional Infinity Fortress first appears when Muzan calls the Lower Moons to a meeting in which he decides to dismantle them, killing them all apart from Lower Moon One, Enmu, who he lets live as his sadistic personality won the Demon lord over.

After the deaths of Gyutaro and Daki, the shared holders of the position of Upper Moon Six, Muzan calls the Upper Moons to a meeting in the fortress.

Later on, during the Dimensional Infinity Fortress Arc, a number of battles take place within the fortress, including Shinobu Kocho vs Doma, Zenitsu Agatsuma vs Kaigaku, Tanjiro Kamado and Giyu Tomioka vs Akaza, Kanao Tsuyuri and Inosuke Hashibira vs Doma, Muichiro Tokito, Genya Shinazugawa, Sanemi Shinazugawa and Gyomei Himejima vs Kokushibo and Demon Slayers vs Muzan Kibutsuji.

During the final battle between Muzan Kibutsuji and the Demon Slayer Corps, Nakime, whose Blood Demon Art controls the fortress, was being manipulated by Yushiro. Muzan discovered this and killed Nakime to put a stop to it, which caused the entire fortress to crumble into pieces.

Trivia

 * Due to Muzan's favoritism towards them, only the Upper Moons of the Twelve Demon Moons are aware of the Fortresses existence and have been regularly called to and summoned by Muzan himself to the fortress for meetings or new orders. The Lower Moons in turn have been implied to never have been summoned or let alone know of the Fortress's existence due to Muzan's own dwindling lack of favor towards them over the centuries due to their constant losses towards the Hashira.