- This article uses algebraic notation to describe chess moves.
In chess, the fortress is an endgame drawing technique in which the side lagging in material sets up a zone of protection around their king that cannot be penetrated by the opponent. Clearly, it only works when the opponent does not have and cannot create a passed pawn, 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:
- Useful pawn breakthroughs are not possible
- If the stronger side has pawns, they are firmly blocked
- The stronger side's king cannot penetrate, either because it is cut off or near the edge of the board
- 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.citation needed
Fortress in a corner
Bishop and wrong rook pawn: draw
Knight protecting rook pawn on the seventh rank: draw
Perhaps the most common type of fortress, often seen in endgames with only a few pieces on the board, is where the defending king is able to take refuge in a corner of the board and cannot be chased away or checkmated by the superior side. These two diagrams furnish two classic examples. In both cases, Black simply shuffles his king between a8 and the available square adjacent to a8 (b8 or b7, as the case may be). White has no way to dislodge Black's king, and can do no better than a draw by stalemate or some other means.
Note that the bishop and wrong rook pawn ending (i.e. where the pawn is a rook pawn whose promotion square is the color opposite to that of the bishop) at right is a draw even if the pawn is on the seventh rank or further back on the a-file. Heading for a bishop and wrong rook pawn ending is a fairly common drawing resource available to the inferior side (Müller & Pajeken 2008:183).
The knight and rook pawn position at right, however, is only a draw if White's pawn is already on the seventh rank, making this drawing resource available to the defender much less frequently. If, for example, White's pawn is moved back to a6, White wins immediately with 1.Kb6! Kb8 2.a7+ and White either checkmates or queens his pawn on the next move.
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Serper-Nakamura
Black to play. White has achieved a fortress.
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A fortress is often achieved by a sacrifice, such as of a piece for a pawn. In the game between Grigory Serper and Hikaru Nakamura, U.S. Chess Championship 2004, White would lose after 1. Nd1 Kc4 or 1. Nh1 Be5 or 1. Ng4 Bg7. Instead he played 1. Nxe4! Kxe4 2.Kf1! heading for h1. After another 10 moves the position in the second diagram was reached. Black has no way of forcing White's king away from the corner, so he played 12... Kf2 and after 13. h4 gxh4 the game was drawn by stalemate.
The back-rank defense in rook and pawn versus rook endings: another type of fortress in a corner.
The back-rank defense in some rook and pawn versus rook endgames is another type of fortress in a corner (see diagram at left). The defender perches his king on the pawn's queening square, and keeps his rook on the back rank (on the "long side" of the king, not, e.g., on h8 in the diagram position) to guard against horizontal checks. If 1.Rg7+ in the diagram position, Black heads into the corner with 1...Kh8! Note that this defense works only against rook pawns and knight pawns.
Rook versus bishop, with the defending king in the "safe" corner. White to move, draw.
In the ending of a rook versus a bishop, the defender can form a fortress in the "safe" corner—the corner that is not of the color on which the bishop resides (see diagram). White must release the potential stalemate, but he can't improve his position (Seirawan 2003:200-201).
- 1. Rc3 Ba2
- 2. Rc2 Bb3
- 3. Rc7 Bg8
de la Villa, 2008
Draw with bishop on either color if the white king does not leave the corner
In this position from de la Villa, White draws if his king does not leave the corner. It is also a draw if the bishop is on the other color (de la Villa 2008:25).
Rook and pawn versus queen
Van der Poel vs. Smits, 2000.
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