It stands to reason that leaving the King in the center often means exposing the King to a dangerous, very possibly fatal, attack. This leads us to the conclusion that castling is the best way to safeguard the King.
The castled position, then, is the King’s safeguard. But, though the King is better protected when castled than when in the center, that does not mean that castling alone assures you complete immunity from attack. If your opponent has an overwhelmingly superior development, he can concentrate more forces for attack than you can supply for defense. Sometimes brilliant sacrifices are made to smash down a defender’s barriers.
But we are now concerned mainly with Pawn weaknesses in the castled position. In the case of castling on the King-side, three Pawns are involved: the King Rook Pawn, the King Knight Pawn, and the King Bishop Pawn. As long as all three Pawns are still on their original squares, the castled position remains strong and difficult to take by storm.
Yet once a single member of the trio advances, the defender is headed for trouble. For example, suppose the King Knight Pawn advances one square. Then immediately the squares it formerly protected–KR3 and KB3–must receive protection from pieces. Worse yet, these squares become targets for enemy occupation. Let a hostile Queen and Knight, or Queen and Bishop, occupy these squares, and you will see the castled position totter and crumble.
The advance of the King Rook Pawn is also dangerously weakening. Very often the attacker is able to sacrifice a piece for the Pawn on KR3, in this way ripping up the castled position and leaving it wide open for large-scale invasion. The advance of the King Bishop Pawn creates similar problems, and very often opens up a vital diagonal for the hostile Bishop.
Another serious consequence of any of these Pawn advances is that they enable the attacker to open lines by advancing his own Pawns and forcing Pawn exchanges. Thus, after Blacks plays . . . P–KN3, White may reply P–KR4 and P–KR5, exchanging Pawns and thus opening the King Rook file for attack. Or, after White plays P–KR3, Black may react with . . . P–KN4 and . . . P–KN5, likewise obtaining an open file for attack.
Once the attacker succeeds in forcing open a line leading to the castled position, he has enormously improved his prospects of taking the hostile King by storm. As long as the Pawns remain on their original squares, they form a road block for the attacking pieces. After one of the Pawns has advanced, the barrier is much more likely to be breached –by exchanges, by sacrifices, by violent line-opening.
To sum up: you have seen that Pawn advances in front of the castled King can be weakening–even dangerous. You should therefore avoid such advances. Sometimes you are forced to make such advances–but at least you can avoid making them needlessly. Avoid such Pawn moves if it is at all possible to avoid them!
Queen-side castling, which we rarely encounter, presents difficulties for the inexperienced player. The castled King has a wider area to guard than on the King-side. Hence the temptation to meet threats with Pawn advances is much stronger in the case of Queen-side castling. This makes it more likely for the defense on this broader front to be upset by violent sacrifices.