Each player may choose to "castle" his king once per match. Castling is the only time a player may move two pieces at one time, and is the only time that a king may move more than one square per turn.
The player moves the king right two spaces, and places the rook on the immediate left of the king. Or the player moves the king left two spaces and places the rook on the immediate right of the king.
There are several pre-requisites for castling.
1. Neither the king nor the rook may have moved during the match.
2. There must be no pieces between the king and the rook.
3. The king must not be in check on the castling move.
4. No space between the king and rook may be threatened by an opposing piece.