Where all the squares between the King and Rook are unoccupied, and
Where no hostile man attacks the square on which the King is to be placed, or the square he crosses.
In Castling, the King must be moved first, or before the Rook is quitted. If the Rook be quitted before the King is touched, the opposing player may demand that the move of the Rook shall stand without the Castling being completed.
The penalty of moving the King prohibits Castling.
EN PASSANT.
Taking the Pawn "en Passant," when the only possible move, is compulsory.
PROMOTING THE PAWN.
A Pawn reaching the eighth square must be at once exchanged for any piece of its own color (except the King) that the player of the Pawn may elect.
CHECK.