CHESSDORRAWhen castling is legal
Castling rights, a clear path, and king safety.
The king’s three squares
The king may not castle out of check, through an attacked square, or onto an attacked square. The squares between the king and rook must be empty.
How castling rights are lost
A king move removes both castling rights. A rook move removes the right to castle with that rook. Returning the piece to its starting square does not restore the right.
Kingside castling
In kingside castling the king moves two squares toward its rook. The rook moves to the square the king crossed. The move is written O-O.