Fortress (chess)

In chess, a fortress is an endgame drawing technique in which the side behind in material sets up a zone of protection that the opponent cannot penetrate. This might involve keeping the enemy king out of one's position, or a safe zone the enemy cannot force one out of (e.g. see the opposite-colored bishops example). An elementary fortress is a theoretically drawn (i.e. a book draw) position with reduced material in which a passive defense will maintain the draw.^[1]

Fortresses commonly have the following characteristics:

1. Useful pawn breakthroughs are not possible.
2. If the stronger side has pawns, they are firmly blocked.
3. The stronger side's king cannot penetrate because it is either cut off or near the edge of the board.
4. Zugzwang positions cannot be forced because the defender has waiting moves available.^[2]

Fortresses pose a problem for computer chess: computers fail to recognize fortress-type positions (unless using a suitable endgame tablebase) and incorrectly claim a winning advantage in them.^[3]