Castling: how and when you can do it
Castling moves the king two squares toward a rook and puts the rook on the other side. It is the only move where two pieces move at once. Try both sides below.
Castling explained
To castle, move the king two squares toward the rook, then move that rook to the square the king crossed. Castling toward the h-file rook is kingside, short, written O-O. Toward the a-file rook is queenside, long, written O-O-O.
It protects the king and brings a rook into the game. Most games include castling within the first ten moves.
What are the rules?
The king and the rook must not have moved before. Squares between them must be empty. The king must not be in check, pass through an attacked square, or land on one.
Can I castle out of check?
No. If your king is in check you must deal with the check first.
Can I castle if my rook is attacked?
Yes. Only the king's path matters. An attacked rook does not stop castling.
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Questions
Can you castle twice?
No. Castling needs the king and that rook to have never moved, so it can happen once per game.
Can a king castle through check?
No. The king may not pass over or land on a square that an enemy piece attacks.
How is castling written?
O-O for kingside and O-O-O for queenside.