Castling: the only move that moves two pieces at once
A move of its own, with rules of its own. Here's everything you need to never get it wrong again.
Key takeaway
Castle early, castle often: a king left in the center too long is your opponent's best friend.
Castling is THE weird move in chess: the king hops two squares and the rook lands right next to it, on the other side. One single move, two pieces moving: that happens nowhere else on the board.
There's kingside castling (short) and queenside castling (long). Kingside is quicker to set up: it only requires clearing two pieces (the bishop and the knight) instead of three.
Three conditions, no more: the king and the rook involved must never have moved during the game, no piece stands between them, and the king neither crosses nor lands on an attacked square.
Watch out for the classic trap: the king can't castle while currently in check, nor pass through an attacked square along the way, even if it doesn't stop there. Many beginners forget this and get the move refused mid-game.
Castling isn't an administrative detail: it's often THE move that decides whether your king survives the game. A king left in the center too long is an open invitation to attack.