Castling is the only move in chess where you move two of your own pieces at once: the king and a rook. It tucks the king away from the centre and brings a rook towards it in a single turn. Below are the castling rules in full, each one checked on a board, and then puzzles from real games where castling is part of the solution.
The king moves two squares towards one of its rooks, and that rook jumps over to the square the king has just crossed. Castling kingside, written O-O, takes White's king from e1 to g1 and the rook from h1 to f1. Castling queenside, written O-O-O, takes the king from e1 to c1 and the rook from a1 to d1. Black castles the same way on the eighth rank.
Castling counts as a king move. Over the board, move the king first and then the rook. In the trainer, castle by moving the king two squares towards the rook.
The king and that rook must never have moved during the game. A king that stepped out and came back has lost the right to castle on both sides for the rest of the game. A rook that moved and came back loses castling on its side only; the other side is still available.
Every square between the king and the rook must be empty. On the queenside that includes the square next to the rook, b1 for White and b8 for Black.
The king cannot castle while it is in check. It cannot pass over a square attacked by an enemy piece, and it cannot land on an attacked square. In short: not out of check, not through check, not into check.
Three things people often believe are not rules. You may castle if you were in check earlier in the game, as long as the king never moved. You may castle when the rook is attacked. And when castling queenside, the rook may cross an attacked square: if b1 is attacked but c1 and d1 are not, White can still play O-O-O, because the king never touches b1.
Castling is usually a quiet, safe move, so it is rarely the point of a tactic. When it is, it tends to be one of three things. It can give check, when the rook lands on an open file or rank facing the enemy king. It can be a defence that gets the king out of the centre just in time. Or it can bring the rook to the file where it is needed for the next move of the combination.
The most famous castling finish comes from a game between Edward Lasker and George Thomas in London in 1912. After a queen sacrifice on h7, White's pieces chased Black's king all the way down the board to g1. In the final position 18.O-O-O is checkmate: the rook arriving on d1 gives check along the first rank, and the king has nowhere to go. The game actually ended with 18.Kd2, a discovered check from the rook on a1 that is mate too, but the castling mate is the one people remember.
The first is believing you can never castle after being checked. The second is thinking an attacked rook stops castling. Both make players give up castling when it was still legal. The opposite mistake also happens: castling into an open file or diagonal, or into a pawn storm, just because castling is usually good. And in puzzles, forget that castling is an option and you will miss the solutions where it is the key.
Castling puzzles are rare, because castling is so seldom the key move of a combination. The trainer gives you the castling puzzles closest to your rating, so they may be easier or harder than your usual puzzles. If you are stuck, the hint shows the piece first, then the move. After you solve, the full line appears beside the board.
For mixed tactics at your level, Puzzle Training is the place to spend most of your time. Puzzle Storm trains speed, the Woodpecker Method repeats a fixed set until it is automatic, and Elo Maxing serves the motif you are weakest at. A castled king behind unmoved pawns is the classic target of a back rank mate, and the other special pawn rule is covered on the en passant page. For opening plans that get the king castled in good time, read the best chess openings.
The king and that rook must not have moved, the squares between them must be empty, and the king cannot be in check, pass over an attacked square or land on one.
Yes, as long as the king has not moved. Being in check only stops you castling at that moment.
Yes. Only the king's squares matter. On the queenside the rook may also cross an attacked square, since the king never passes b1 or b8.
Kingside castling, O-O, moves the king to the g-file and the rook to the f-file. Queenside castling, O-O-O, moves the king to the c-file and the rook to the d-file.
Yes. If the rook lands on a line to the enemy king and the check cannot be answered, castling is checkmate, as in the final position of Lasker against Thomas, London 1912.