Castling is a special defensive maneuver. It is the only time in the game when more than one piece may be moved during a turn.

This move was invented in the 1500's to help speed up the game and to help balance the offense and defense. It can only occur if there are no pieces standing between the king and the rook; neither king nor rook may have moved from its original position; there can be no opposing piece that could possibly capture the king in his original square, the square he moves through or the square that he ends the turn.

The king moves two squares toward the rook he intends to castle with - this may be either rook. The rook then moves to the square through which the king passed.