What is the name of the optional dungeon in Final Fantasy VI that contains the Esper Odin?

The optional dungeon in Final Fantasy VI that contains the Esper Odin is the Ancient Castle.

The Ancient Castle is hidden underground between Kohlingen and South Figaro and becomes accessible after the party obtains the airship Falcon in the World of Ruin. Figaro Castle’s underground engine accidentally reaches the cavern leading to the ruins, turning the discovery into an optional exploration area rather than a required story destination.

The castle is tied to the War of the Magi, a conflict that occurred roughly 1,000 years before the game. Its guardian, the Esper Odin, was petrified while defending the castle from a powerful sorcerer. In the throne room, the party can inspect Odin’s remains to obtain the Odin magicite. A hidden passage leads to the petrified queen, whose story can transform Odin into the Esper Raiden in most versions.

The Ancient Castle is often confused with Cultists’ Tower or Kefka’s Tower, but those locations serve different purposes: Cultists’ Tower is associated with the Fanatics and a difficult vertical challenge, while Kefka’s Tower is the game’s endgame dungeon.