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 1500s to help speed up the game and to help balance the offense and defense.
The castling move has some fairly rigid restrictions:
- It can only occur if there are no pieces standing between the king and the rook.
- Neither king nor the rook to be castled with may have moved from its original position. (The other rook may have already moved.)
- 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. Hopefully, the diagram to the left makes this clear. If the king castles queen-side the rook must move three squares. However on the king-side the rook only moves two squares.