Infinity Castle

The Infinity Castle, also known as the Dimensional Infinity Fortress, was the lair of the Twelve Kizuki and the primary location of the Infinity Castle Arc.

History
The Infinity Castle first appears when Muzan calls the Lower Ranks to a meeting in which he decides to dismantle them, killing them all apart from Lower Rank 1, 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 Rank 6, Muzan calls the Upper Ranks to their first meeting in 113 years, in the fortress.

Then, after the deaths of Gyokko and Hantengu, as well as the revelation of Nezuko being immune to the Sun, Muzan gathers all of the demons in the country and turns them into mindless demons with the strength of a lower-ranked demon moon. These demons would later be seen as cannon fodder to be used against the various members of the Demon Slayer Corps.

Later on, during the Infinity Castle 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: Part 1.

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 Ranks of the Twelve Kizuki are aware of the castle's existence and have been regularly called to and summoned by Muzan himself to the castle for meetings or new orders. The Lower Ranks in turn have been implied to never have been summoned or let alone know of the castle's existence due to Muzan's own dwindling lack of favor towards them over the centuries due to their constant losses towards the Hashira.