Castle Early

Castling is the only move that improves two pieces at once: the king ducks behind a pawn wall, and a rook wakes up and heads for the center files.

The default: castle by move 10. A king still standing on e1 or e8 while the center opens up is where miniature disasters come from. Let's watch one — a 300-year-old trap called Légal's Mate that still catches thousands of players every day.