Fortress (chess)

Fortress (chess)

In chess, the fortress is an endgame drawing technique in which the side behind in material sets up a zone of protection around their king that cannot be penetrated by the opponent. This only works when the opponent does not have a passed pawn or cannot create one, unless that pawn can be stopped (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 (Müller & Pajeken 2008:183).

Fortresses commonly have four 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, either because it is cut off or near the edge of the board
4. Zugzwang positions cannot be forced, because the defender has waiting moves available (de la Villa 2008:23).

Fortresses pose a problem for computer chess: computers are unable to reason about fortress-type positions except to the extent that their endgame tablebase allows.