Lesson 5 - The Rule of Castling in Chess

"Castle early and often"
- Rob Sillars

Since the goal in chess is to checkmate the King, it is logical that this royal piece is victim of several threats coming from the enemy.  It is then of the utmost importance to take care of its safety by hiding it behind its teammates and ensuring that the squares around it are well protected. There is a special move available to both players to help fulfilling these objectives: the "castling".


Why castling is a special move?

Well, it is because the move goes against several rules.  Indeed, when castling:

  • the King is moving two squares instead of one;
  • two pieces are played on the same move (the King and a rook);
  • it allows the rook to pass over another piece (the King).


Why castling is useful?

Castling offers the players a fast way of putting the King in a safe spot near the side of the board, far from the action usually occuring in the center columns. The center files are often opened and dangerous during the first phases of the game.  Moreover, castling allows the rook, which is confined behind the other pieces, to play a more active role in the game.  To help you understand the value of this move, let's consider the following diagram:

We have to shelter the King

Diagram 5.1 - We have to shelter the King

An excellent strategy for White in this position would be to move the King to the g1 square so it would be sheltered by its pawns.  By moving the King on this square, the pawns and the King will protect themselves mutually. White could take two moves in order to put the King on g1:

Move the King to g1

Diagram 5.2 - Move the King to g1

However, this strategy would confine the rook to a very passive role.  White should then allow the rook to quit its starting square before moving the King to g1.  The maneuver will then take four moves:

  1. Move the king to e2

  2. Move the rook to e1

  3. Move the king to f1

  4. Move the king to g1
4 moves to put the king to safety

Diagram 5.3 - We need 4 moves to put the king to safety

After all these moves, White has a much better position.  But four moves are a lot... and in a real game, his opponent will try his best to prevent White to succeed in reaching this position. If we think about it, both players would benefit to have this setup... thus castling has been introduced in order to speed up the game.


Ok... very useful move but how do we do it?

So here we are.  How are we castling?  Casting is considered as a King move. The player must take the King and moves it horizontally two squares (left or right). Afterward, the player must take the rook he will find on this side of the board and move it to the first square on the other side of the King. The rook is then passing over the King. No piece must occupy the squares in between the King and the rook prior to execute the move.

If we have another look at the position we used in diagram 5.1, we can see that White could fulfill his objective in only one move by castling:


Diagram 5.4 - Castling

In the above diagram, White is castling on the kingside. We may say also that White is "castling short" since only two squares are in between the King and the rook. When a player is castling on the queenside, three squares separate the King and the rook so we can also say that the player is "castling long". Here is an example of castling queenside:

Castling Long

Diagram 5.5 - Castling long

Note that when castling queenside, the King does not protect all three pawns as it is the case when castling on the kingside. Indeed, the a pawn is no longer protected after the castling. Pay attention to this pawn when castling long. If it is under attack before you execute your castling, you risk loosing it and the king will no longer have a strong shield. The queenside is a bit more risky for the King but it benefits to the rook since it will occupy a central file allowing for a quick entry in the action. The rook does not usually have this chance when castling on the kingside. In certain positions, bringing the rook on the d file when castling queenside can be very interresting.


Special conditions

No doubt you are starting to appreciate the virtues of this move: it has the power to transform a position in a blink of an eye. For this reason, some restrictions were defined to limit its accessibility:

  • no piece must stand in between the king and the rook a player wants to use for castling;
  • both pieces must occupy their starting square and must not have been moved since the beginning of the game;
  • the king must not be in check;
  • the king must never cross a square controlled by an enemy piece;

The last condition might need a bit more explanation. For example, if Black is controlling the f1 square, White cannot castle kingside since his King must cross the controlled f1 square to reach g1. It might be easier to understand this condition with a picture:

White cannot castle kingside since black's bishop is controlling the f1 square

Diagram 5.6 - White cannot castle kingside since black's bishop is controlling the f1 square

The main objective of castling is to put the king in a safety area during the most violent phases of the game. That's the reason why this move is usually done early in the game.  In most cases, castling is done prior to the fifteenth move.

In high level games, the side of the castling has a big influence on the development strategies of both players. Some players will even wait as long as they can before castling in order to keep their opponent in the mystery.

Let's continue the lesson with the next page on drawn gamesonline chess tutorial.