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.
[1] Fortresses commonly have the following characteristics: 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.
Heading for a bishop and wrong rook pawn ending is a fairly common drawing resource available to the inferior side.
After another 10 moves the position in the following diagram was reached: Black has no way of forcing White's king away from the corner, so he played and after 13.h4 gxh4 the game was drawn by stalemate.
[8] In this position from de la Villa, White draws if their king does not leave the corner.
[12] In this 1959 game between Whitaker and Ferriz, White sacrificed a rook for a knight in order to exchange a pair of pawns and reach this position, and announced that it was a draw because (1) the queen cannot mate alone, and (2) the black king and pawn cannot approach to help.
[13] However, endgame tablebase analysis shows Black to have a forced win in 19 moves starting with 50... Qc7+ (the only winning move), taking advantage of the fact that the rook is currently unprotected – again illustrating how tablebases are refining traditional endgame theory.
From the diagram, in Salov vs. Korchnoi, Wijk aan Zee 1997,[14] White was able to hold a draw with a rook versus a queen, even with the sides having an equal number of pawns.
For example:[18] In the two bishop versus queen ending, the queen wins if the Lolli position is not reached, but some of them take up to seventy-one moves to either checkmate or win a bishop, so the fifty-move rule comes into play.
In Ree-Hort, Wijk aan Zee 1986 [22] (first diagram), Black had the material disadvantage of rook and bishop against a queen.
White's queen has no moves, all of Black's pawns are protected, and his bishop will shuttle back and forth on the squares a1, b2, c3, and d4.
However, to do so Black has to move his king so far from the pawn that White can play Ka3–b2 and Nc5xb3, when the rook versus knight ending is an easy draw.
Unlike other forms of fortress, a defense perimeter can often be set up in the middlegame with many pieces remaining on the board.
White already has a huge material disadvantage, but forces a draw by giving up their remaining pieces to establish an impenetrable defense perimeter with their pawns.
Now Black is up two rooks and a bishop (normally an overwhelming material advantage) but has no hope of breaking through White's defense perimeter.
[27] The above example may seem fanciful, but Black achieved a similar defense perimeter in Arshak Petrosian–Hazai, Schilde 1970[28] (first diagram) via a swindle.
Black has a difficult endgame, since White can attack and win his a-pawn by force, and he has no counterplay.
This is actually a critical mistake, enabling Black to establish an impenetrable fortress.
46... cxb6 Now Black threatens 47...h4, locking down the entire board with his pawns, so White tries to break the position open.
Now Smirin gives HIARCS the choice between an opposite-colored bishops endgame (in which, moreover, White will play Be7 and win the h-pawn if Black's king comes to the center) and a bishop versus knight ending in which Smirin envisions a fortress.
The endgame of two bishops versus a knight was thought to be a draw for more than one hundred years.
Computer endgame tablebases show that the bishops generally win, but it takes up to 66 moves.
[33] This game[34] between József Pintér and David Bronstein demonstrates the human play of the endgame.
[35] A "positional draw" is a concept most commonly used in endgame studies and describes an impasse other than stalemate.
It usually involves the repetition of moves in which neither side can make progress or safely deviate.
[40] This position from a game between Mikhail Botvinnik and Paul Keres in the 1951 USSR Championship is drawn because the black king cannot get free and the rook must stay on the c-file.
The position looks lost for White, as he cannot stop the h-pawn from queening, but he does have a defence which seems to defy the rules of logic.
White threatens to stop the advance of the h-pawn with ...Be5+; building the fortress immediately does not work: 1.f6?
Chess computer programs have difficulty assessing "fortress" positions because the normal values for the pieces do not apply.
Now White can build his "fortress" without the worry of the queen getting to the back rank via the long diagonal.