The Ancient Castle of Ikana is a mini-dungeon from The Legend of Zelda: Majora's Mask. This castle has served as the ruling place of Igos du Ikana and the rest of the Ikana Royal Family of the Kingdom of Ikana for untold ages. In the throne room of the castle, Link does battle with the king and his bodyguards.
On his quest to halt the impending destruction of Termina, Link arrives in the desolate Ikana Canyon. Link learns of the curse placed on the area, and frees Sharp, one of the Composer Brothers from the curse by playing a song that he learned from his brother, Flat. Link restores the flow of water in the canyon and reactivates the Music Box House where Pamela and her cursed father reside; the song it emits, "Farewell to Gibdos", is hated by the Gibdos in the area, and they burrow under the ground whenever the song plays. Link enters the house and heals the soul of Pamela's father with the "Song of Healing". The Gibdo curse that was eating away at Pamela's father is turned into a Gibdo Mask, which Link uses to communicate with the Gibdos living in the nearby Well. By trading them items that they long for, Link works his way through the Well and finds the Mirror Shield. Using this, he finds an exit from the well leading to the grounds of the Ancient Castle of Ikana.
Link enters the Castle and solves many puzzles, eventually opening the door to the throne room, where Igos du Ikana and his bodyguards reside. Driven mad by the curse, they attack Link. Link uses his Fire Arrows to destroy the curtains keeping the light out of the room, and uses the Mirror Shield to deflect light upon Igos' bodyguards, who are defeated. Igos himself then challenges Link to a duel, and falls before Link. Now freed from the curse, the king thanks Link for ending their suffering and teaches him the "Elegy of Emptiness", a mysterious tune that creates a copy of each of Link's different forms, allowing him to climb the Stone Tower.
In order to penetrate the walls of the Ancient Castle of Ikana, Link must first hit a Crystal Switch 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 interior of the castle is quite dark, as its inhabitants appear to hate the light, as can be demonstrated when Link fights Igos du Ikana and his bodyguards, and 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, which upon being destroyed brings light into the room which Link reflects with his Mirror Shield.)
Several ReDeads also inhabit this castle, and thus, various masks such as the Captain's Hat, Gibdo Mask, or Garo's Mask (wearing one of these will cause them to dance) are useful in allowing for safe passage, as some rooms contain up to four or more ReDeads.
Beneath the Well
The official Majora's Mask strategy guide states that the Ancient Castle of Ikana is home to a countless number of ReDeads and Wallmasters. Despite this however, not a single Wallmaster is encountered in the mini-dungeon. This can lead one to believe that the Ikana Well is actually considered part of the Ancient Castle of Ikana, rather than a seperate mini-dungeon that just happens to be connected.
This statement could also be explained as simply a misprint or error. It could also be that maybe Wallmasters are in the Castle, but Link just happens to never come across one or that it is actually meant to refer to Floormasters.