There are two types of castling: Kingside and Queenside. Either way, the King is moved two squares toward the Rook, and then the Rook is moved to the square through which the King passed. In chess notation, castling Kingside is written 0-0, and Queenside is 0-0-0.
White castled Queenside, and his opponent castled Kingside.
Castling is prohibited if::The King is in check | |
The King passes through check | |
The King ends up in check | |
The King has already moved | |
The Rook that is used has moved |