Fortress (chess)

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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).

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

Contents 1 Defense perimeter (pawn fortress) 2 Fortress in a corner 3 Rook and pawn(s) versus queen fortress 4 Fortress with opposite-colored bishops 5 Fortresses with a queen versus two minor pieces 6 Other fortresses 7 See also 8 References 9 External links

[edit] Defense perimeter (pawn fortress)

The defense perimeter is a drawing technique in which the side behind in material or otherwise at a disadvantage sets up a perimeter, largely or wholly composed of a pawn chain, that the opponent cannot penetrate. Unlike other forms of fortress, a defense perimeter can often be set up in the middlegame with several pieces remaining on the board.

The position at left, taken from a chess problem, illustrates the defense perimeter. White already has a huge material disadvantage, but forces a draw by giving up his remaining pieces to establish an impenetrable defense perimeter with his pawns. White draws with 1.Ba4+! Kxa4 (1...Kc4 2.Bb3+! Kb5 3.Ba4+ repeats the position) 2.b3+ Kb5 3.c4+ Kc6 4.d5+ Kd7 5.e6+! Kxd8 6.f5! (diagram at right). 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. The only winning attempts Black can make are to place his rooks on b5, c6, etc. and hope that White takes them. White draws by ignoring all such offers and simply shuffling his king about.

The above example may seem fanciful, but Black achieved a similar defense perimeter in Arshak Petrosian-Hazai, Schilde 1970[1] (diagram at left), which Soltis discusses in his book The Art of Defense in Chess. In the position at left, Black has a difficult endgame, since White can attack and win his a-pawn by force, and he has no counterplay. Black tried the extraordinary 45...Qb6!?, to which White replied with the obvious 46.Nxb6+? This is actually a critical mistake, enabling Black to establish an impenetrable fortress. White should have carried out his plan of winning Black's a-pawn, for example with 46.Qc1 (threatening 47.Nxb6+ cxb6 48.h4! gxh4 49.Qh1 and Qh3, winning) Qa7 47.Qd2 followed by Kb3, Nc3, Ka4, and Na2-c1-b3. 46...cxb6 Now Black threatens 47...h4, locking down the entire board with his pawns, so White tries to break the position open. 47.h4 gxh4 48.Qd2 h3! 49.gxh3 Otherwise 49...h2 draws. 49...h4! (diagram at right) Black has established his fortress, and now can draw by simply moving his king around. The only way White could attempt to breach the fortress would be a queen sacrifice at some point (for example Qxa5 or Qxe5), but none of these give White winning chances as long as Black keeps his king near the center. The players shuffled their kings, and White's queen, around for six more moves before agreeing to a draw.

In Smirin-HIARCS, Smirin-Computers match 2002,[2] the super-grandmaster looked to be in trouble against the computer, which has the bishop pair, can tie White's king down with ...g3, and threatens to invade with its king on the light squares. Smirin, however, saw that he could set up a fortress with his pawns. The game continued 46...g3 47.h3! A surprising move, giving Black a formidable protected passed pawn on the sixth rank, but it begins to build White's fortress, keeping Black's king out of g4. 47...Bc5 48.Bb4! Now Smirin gives HIARCS the choice between an opposite-colored bishop ending (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. 48...Bxb4 49.axb4 Kf7 Black could try to prevent White's coming maneuver with 49...Bd3, but then White could play 50.Nf3 Kh5 (forced) 51.Nd4. 50.Nb5! Ke6 51.Nc3! Completing the fortress. Now Black's king has no way in, and his bishop can do nothing, since White's King can prevent ...Bf1, attacking White's only pawn on a light square. The game concluded: 51...Bc2 52.Kg2 Kd6 53.Kg1 Kc6 54.Kg2 b5 55.Kg1 Bd3 56.Kg2 Be4+ 57.Kg1 Bc2 58.Kg2 Bd3 59.Kg1 Be4 60.Kf1 1/2-1/2

[edit] Fortress in a corner

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. The diagrams at left and right furnish two classic examples. In both cases, Black simply shuffles his king between a8 and b7. White has no way to dislodge Black's king, and can do no better than stalemate Black.

Note that the bishop and wrong-colored rook pawn ending (i.e. where the pawn promotes on the color opposite to that of the bishop) at left is a draw even if the pawn is further back on the a-file. Heading for a bishop and wrong-colored rook pawn ending is a fairly common drawing resource available to the inferior side.

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 mates or queens his pawn on the next move.

Grigory Serper versus Hikaru Nakamura. White to play.

Black to play. White has achieved a fortress.

A fortress is often achieved by a sacrifice, such as of a piece for a pawn. In Serper-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 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.

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

[edit] Rook and pawn(s) versus queen fortress

In the diagram at right, Black draws by moving his rook back and forth between the e6 and g6 squares, or moves his king when checked, staying behind the rook and next to the pawn. This fortress works when all of these conditions are met:

1. the pawn is still on its second rank
2. the pawn is on files b through g
3. the pawn is protecting its rook on the third rank
4. the opposing king is beyond the defender's third rank
5. the defending king protects its pawn (Snape 2003:91).

The white king is not able to cross the rank of the black rook and the white queen is unable to do anything useful.

• 1. Qd5+ Rd6
• 2. Qb5+ Kd8
• 3. Qb8+ Kd7
• 4. Qb5+ draw

From the diagram at right, in Salov-Korchnoi, Wijk aan Zee 1997,[3] White was able to hold a draw even with equal pawns. He kept his rook on the fifth rank blocking in Black's king, and was careful not to lose his rook to a fork or allow a queen sacrifice for the rook in circumstances where that would win for Black. The players agreed to a draw after:

48.Kg2 Kg6 49.Rh5 Qe2+ 50.Kg3 Qf1 51.Kf4 Qe1 52.Rd5 Qc1+ 53.Kg3 Qc7+ 54.Kg2 Qf4 55.Rh5 Kf6 56.Rd5 Ke6 57.Rh5 Qd2+ 58.Kg3 f6 59.Rf5 Qc1 60.Rh5 Qg1+ 61.Kf4 Qe1 62.Rb5 Qc1+ 63.Kg3 Qg1+ 64.Kf4 Qh2+ 65.Ke3 Kf7 66.Rh5 Qg1+ 67.Kf4 Kg6 68.Rd5 Qh2+ 69.Ke3 Kf7 70.Rh5 Qg1+ 71.Kf4 Ke6 72.Rb5 Qh2+ 73.Ke3 Kd6 74.Rf5 Qb2 75.Rh5 Ke6 76.Kf4 Qc3 77.Kg3 Qc7+ 78.Kg2 Qf7 79.Rb5 Qe8 80.Rf5 Qg6 81.Rb5 1/2-1/2

[edit] Fortress with opposite-colored bishops

This is an example of a drawing fortress with opposite-colored bishops when three pawns behind. White simply keeps his bishop on the h3 to c8 diagonal (Dvoretsky 2003:91).

[edit] Fortresses with a queen versus two minor pieces

Here are drawing fortresses with two minor pieces versus a queen (Dvoretsky 2003:272). Usually the defending side will not be able to get to one of these positions.

The bishop and knight fortress is another type of fortress in a corner. If necessary, the king can move to one of the squares adjacent to the corner, and the bishop can retreat to the corner. This gives the inferior side enough tempo moves to avoid zugzwang. For example: (Müller and Lamprecht 2001:339-41)

• 1. Kb5 Ka7
• 2. Qd8 Ba8
• 3. Ka5 Bb7.

In the two bishop versus queen ending, the queen wins if the Lolli position isn't reached, but some of them take up to seventy-one moves, so the fifty move rule comes into play. From the diagram:

• 1. Qe7+ Kc8
• 2. Qe6+ Kb7
• 3. Qd6 Ba7
• 4. Qe7+ Kb6!
• 5. Qd8+ Kb7!
• 6. Ka5 Bc5!

and white can't prevent ... Bb6, which gets back to the Lolli position (Müller and Lamprecht 2001:339-41).

In the two knights fortress, the knights are next to each other and their king should be between them and the attacking king. The defender must play accurately, though (Müller and Lamprecht 2001:339-41).

There are several drawing positions with two knights against a king. The best way is to have the knights adjacent to each other orthogonally with their king between them and the enemy king. This is not a true fortress since it isn't static. The position of the knights may have to change depending on the opponent's moves. In this position (Lolli 1763),

• 1. Qd1 Nd2+
• 2. Ke2 Nb3

and Black has an ideal defensive position.

If the knights cannot be adjacent to each other orthogonally, the second best position is if they are next to each other diagonally (see diagram).

The third type of defensive formation is with the knights protecting each other, but this method is more risky (Nunn 2002:300ff).

[edit] Other fortresses

At the great New York 1924 tournament, former world champion Emanuel Lasker was in trouble against his namesake Edward Lasker, but surprised everyone by discovering a new endgame fortress.[4] White draws despite having only a knight for a rook and pawn by moving his knight back and forth between b2 and a4. Black's only real winning try is to get his king to c2. 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. The game concluded:

93.Nb2 Ke4 94.Na4 Kd4 95.Nb2 Rf3 96.Na4 Re3 97.Nb2 Ke4 98.Na4 Kf3 99.Ka3! Ke4 If 99...Ke2, 100.Nc5 Kd2 101.Kb2! (101.Nxb3+?? Kc2-+) and 102.Nxb3 draws. 100.Kb4 Kd4 101.Nb2 Rh3 102.Na4 Kd3 103.Kxb3 Kd4+ 1/2-1/2

[edit] See also

• Chess terminology
• Endgame
• Swindle (chess)

[edit] References

•   Dvoretsky, Mark (2003). Dvoretsky's Endgame Manual. Russell Enterprises. ISBN 1-888690-19-4. 

•   Karsten Müller and Frank Lamprecht (2001). Fundamental Chess Endings. Gambit Publications. ISBN 1-901983-53-6. 

•  John Nunn (2002). Secrets of Pawnless Endings. Gambit Publications. ISBN 1-901983-65-X. 

•   Yasser Seirawan (2003). Winning Chess Endings. Everyman Chess. 1-85744-348-9. 

•   Snape, Ian (2003). Chess Endings Made Simple: How to Approach the Endgame with Confidence. Gambit Publications. ISBN 1-901983-97-8. 

[edit] External links

• Serper vs. Nakamura 2004 (requires java)