The Castle (Japanese: 霧の城 Kiri no Shiro, lit. "Castle of the Mist") is the setting of Ico.
Built atop four tall rocky islands, it is virtually desolate, inhabited only by the Queen, her daughter, and her shadowy minions. Whoever else once lived there has long since moved on, and the castle has since begun falling into ruin. Within the castle there are various mechanisms, from the Idol Gates, to staircases and even invisible walkways, which only the power Yorda possesses can activate. She shares this power with a sword that Ico eventually gains use of, and devices above the castle's main gates. The latter are part of the largest of these mechanisms, spanning three of the four islands and apparently responsible for securing the main gates. It is using this mechanism to open the way out which makes up the bulk of Ico and Yorda's journey.
The degradation of the castle reflects the state of its dying monarch. When the Queen finally dies the castle dies with her, consigned to a watery grave; The Queen's power may have been the only thing preventing it from doing so all along.
The central island holds the bulk of the castle. It consists of a highly irregular mishmash of buildings, courtyards, gardens, and some standout landmarks such as the windmill, waterfall and the castle's main gates. The island on which it is built is no more regular; below rooms with damaged floors there is nothing but empty space, and there are small bridges in various locations which cross sheers drops of hundreds of feet between the adjoining buildings. There is even a large cleft which runs roughly across the middle of the castle, which has few means of crossing it.
As with the auxiliary islands, the built up area extends to the very edge of the island; look over a wall at the extreme reaches of the castle and all you will see below is ocean.This island also features a cave system which can be accessed from certain parts of the castle. The caves contain waterfalls whose energy is harnessed by a complex and massive series of water wheels, gears and axles. The use of this energy is not entirely clear. This area is notably less solidly constructed; in place of the stonework of the castle above, there are wooden walkways, broken in places. The east and west islands, mirror images of each other, hold the arenas and reflectors. The reflectors are used to open the main gate by shining a beam of light the devices above it. For these beams to reach the gate however, three circular doors in each arena must be opened. Each island can be accessed by means of a massive bridge leading from the castle.
The final island is to the north, at the rear of the castle proper. It houses the Queen's throne room, the chamber in which Ico was entombed, and the tower in which Yorda was caged. A dock in a natural cave at the base of the island provides a second way into the castle, the route used by the men who brought Ico to the castle, and subsequently by Yorda to free him. Access to the dock is provided by a lift which leads directly to the chamber Ico was entombed in. This chamber and the throne room were apparently meant to be quite secure, as the route from there to the main island involves some climbing and appears to be one way, and the lift is secured not only with Idol Gates, but also by the floor raising to conceal the doorway in the upper chamber.