What is castling, and when should you do it?
The only move where two pieces move at once. Here are the four conditions, and the judgement call about which side to castle.
Castling is the one move in chess where two pieces move at the same time. The king travels two squares towards a rook, and that rook jumps over to the square the king passed. It exists for one reason: to get the king out of the centre before the position opens.
The four conditions
Castling is legal only if all four are true.
- Neither the king nor that rook has moved. Moving a rook and putting it back does not reset this.
- The squares between them are empty.
- The king is not currently in check.
- The king does not pass through, or land on, a square that is attacked.
Note what is not on the list: the rook may pass through an attacked square, and the rook may be attacked. Only the king's journey matters. This surprises almost everyone.
Kingside and queenside
Kingside (short castling) moves the king two squares towards the h-file rook. It is quicker, needs only two squares cleared, and leaves the king tucked behind three untouched pawns.
Queenside (long castling) moves the king towards the a-file rook and needs three squares cleared. The king ends a little more exposed, but the rook lands on a central file immediately, which is often worth more.
When to castle
Early. Most strong players castle within the first ten moves. Beginners leave it too late and get punished when a central file opens.
The main reason to delay is if you are certain the position will stay closed, or if you are not yet sure which side is safer.
Which side
A rough guide:
- Castle on the side where your pawns are intact. A king behind advanced pawns has no shelter.
- Castle away from the opponent's attack. If they have pieces massed on the kingside, going queenside removes the target.
- Opposite-side castling means a race. If you castle queenside and they castle kingside, both sides throw pawns at each other's king, and whoever arrives first usually wins. It is sharp and not for the faint-hearted.
The most common error
Castling into an attack. A child sees "castle early" as an absolute rule and castles kingside directly into three enemy pieces aimed at h7. The rule is a default, not a law. Look at what is pointing at the square first.
The second most common error
Moving the king or rook "just for one move" and losing castling rights forever. There is no way to get them back.
Castling, with the traps around it, is covered in month three of our Beginner course.