The Ancient Castle of Ikana is the ruling place of Igos du Ikana in the kingdom of Ikana in The Legend of Zelda: Majora's Mask. It is ancient, half destroyed, and sealed shut, but Link accesses it through the bottom of the well. The castle is a mini-dungeon in the game. In the end, Link fights Igos and his elite guard, and after defeating them, learns the Elegy of Emptiness.
In order to reach the castle, Link must first hit a crystal and use the Mirror Shield to remove a sun block from the entrance to the castle courtyard. He must then hit an iced-over switch with a Fire Arrow, allowing access into the Ancient Castle. The castle is also quite literally dark, as the creatures inside appear to hate the light, as can be demonstrated when Link fights Igos du Ikana and his guards when he destroys them with light after burning the throne room's curtains. The Mirror Shield is a critical item to progress through the castle, as several areas require that he use light to proceed into the temple. (At times, light is blocked from the castle and thus Link must bring more into the castle. For example, in one part of the dungeon Link must go outside and plant a bomb on a stone panel, upon destroying which brings light into the room which Link reflects with his Mirror Shield.)
Several ReDeads also inhabit this castle. Thus, the Sun's Song and various masks such as the Captain's Hat, Gibdo Mask, or Garo Mask (wearing one of these will cause them to dance) are useful in allowing for safe passage, as some rooms contain up to 4 or more ReDeads.