Castle early

To castle early means trying to use castling during the first part of the game once it is legal and sensible. Castling is a special move: the king moves two squares toward a rook, and that rook moves to the square the king crossed. In one move, the king usually leaves the center and a rook moves closer to the central files.

No exact move number defines early. The principle is meant to prevent the center from opening while the king is still there. Preparing kingside castling normally requires developing moves that clear the knight and bishop from the squares between king and rook.

Legal and sensible

Castling Is Available and Sensible

8

7

6

5

4

3

2

a1

b

c

d

e

f

g

h

Start

O-O

Everything is prepared

The king and rook retain their castling rights, f1 and g1 are clear, and Black does not attack e1, f1, or g1.

When castling is legal

• The king and the chosen rook must not have moved earlier.
• No piece may stand between that king and rook.
• The king may not be in check, cross an attacked square, or finish on an attacked square.

Unavailable

Appearance Does Not Preserve Castling Rights

8

7

6

5

4

3

2

a1

b

c

d

e

f

g

h

Start

Rg1

Bh3

Rh1

The right exists, but the route is attacked

White still has kingside castling rights, but the black bishop on g2 attacks f1. O-O is not legal while the king would have to cross that attacked square.

This is a principle, not an automatic command. Delaying castling can be correct if the opponent already has a strong attack on that wing, the center is closed, or an immediate tactic demands attention. Spending too many moves just to force castling can also be dangerous. In many positions castling later helps connect the rooks, but the king's actual safety comes first.

Legal but unsafe

Legal Does Not Mean Safe

8

7

6

5

4

3

2

a1

b

c

d

e

f

g

h

Start

O-O

Qxh2#

The rules allow castling

The pawn on f2 blocks the queen's diagonal toward e1, and e1, f1, and g1 are not attacked. Kingside castling is legal, but the queen on h4 and bishop on d6 are already focused on h2.