Castling

Castling tucks the king away and brings a rook into play, in a single move. The king steps two squares towards the rook, and the rook hops over him to land right beside him.

There are conditions: neither the king nor that rook may have moved already, the squares between them must be empty, and the king must not be in check — nor pass through a square that is attacked.

Castle early. A king left in the centre almost always comes to regret it.