Fortress

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Fortress, an endgame drawing technique where a side usually behind in material sets up a zone of protection that the opponent cannot penetrate. While fortresses are quite common in (late) endgames, f.i. in Wrong Color Bishop and Rook Pawn, fortresses may rarely occur in the (late) middlegame, characterized by long fixed pawn chains with up to one or two open files but all points of penetration well defended by the weaker side. Chess programs without the implemetation of special knowledge typically fail to recognize fortresses and seem to claim a winning advantage, although they are not able to achieve the win against adequate defence.

=Fortress Detection=

Evaluation
Fortress detection might be triggered in evaluation or interior node recognizers if certain material configurations along with the winning advantage, and pawn structure properties occur. Further pattern matching considering relevant features of fortress positions or fuzzier pattern recognition is then used to scale the winning advantage towards the draw score zero - the fuzzier and possibly more error-prone the heuristics, the less the draw scaling. However, due to search versus knowledge trade-off, and all that generalization and tuning problems involved with adding knowledge and noise, practical playing strength versus usefulness in analysis is an issue.

Sample
A pattern of rook vs queen fortress in the endgame was given by Lyudmil Tsvetkov in his 2017 The Secret of Chess : One side having just one queen, and at most a single pawn, with the other having just one rook, and at least one pawn, if the queen side has no passers, the rook is protected by a pawn on the g or b files on its 2nd rank, the king of the rook side is adjacent to that pawn, the pawn of the queen side is on files h,f,a or c, and the king of the queen side is not past its 5th rank.

Search
Another technique of fortress detection as proposed by Matej Guid and Ivan Bratko in 2012 is related to search, in particular iterative deepening along with "no progress" indication - that is the root evaluation or low search depths already indicate a winning score, which is not increasing in consecutive iterations. A few programs start scaling scores toward zero if the halfmove clock already exceeds values far below enforcing the 50 move rule, often triggered by conditions mentioned above. Matej Guid and Ivan Bratko further mention the possibility of Monte-Carlo Tree Search to detect fortresses.

=See also=
 * Bishops of Opposite Colors
 * Blockade
 * Blockage Detection
 * Draw
 * Draw Evaluation
 * Draw Scores
 * Fortress (Engine)
 * Insufficient Material
 * Pattern Recognition
 * Perpetual Check
 * Repetitions
 * Stalemate
 * Wrong Color Bishop and Rook Pawn

=Selected Publications=
 * Karsten Müller, Wolfgang Pajeken (2008). How to Play Chess Endings. Gambit Publications, amazon, Chapter 11
 * Lyudmil Tsvetkov (2010). Little Chess Evaluation Compendium. 2010 pdf
 * Matej Guid, Ivan Bratko (2012). Detecting Fortresses in Chess. Elektrotehniški vestnik, Vol. 79, Nos. 1-2, pdf » Rybka, Houdini
 * Lyudmil Tsvetkov (2017). The Secret of Chess. amazon

=Forum Posts=

2000 ...

 * Thanks -- and a question about "fortress" recognition in programs by Steve, CCC, March 21, 2002
 * The Bahrain Fortress by Mike Scheidl, December 10, 2002 » Kramnik versus Deep Fritz 2002 - Game 6
 * missing a win or fortress? by Uri Blass, CCC, July 21, 2004

2005 ...

 * Fortress: is there an engine that can solve this? by Peter Kasinski, CCC, December 18, 2005
 * Is this positions drawn ? by M. Ansari, CCC, May 18, 2007
 * Fortress detection program by Anil, CCC, August 19, 2008
 * Fortress Draws by Ted Summers, CCC, May 31, 2009

2010 ...

 * Funny fortress position by Joona Kiiski, CCC, January 25, 2010
 * "No progress" and Graph History Interaction by BB+, OpenChess Forum, January 23, 2011 » Graph History Interaction
 * Another fortress position by Robert Flesher, CCC, March 13, 2011
 * Simple fortress detection, engines fail by Vincent Lejeune, CCC, July 04, 2012
 * test position: fortress by Jon Dart, CCC, December 02, 2012
 * Idea for recognizing fortress/encouraging progress by Piotr Lopusiewicz, CCC, May 03, 2013
 * Re: Chessprogams with the most chessknowing by Ferdinand Mosca, CCC, December 20, 2014

2015 ...

 * Endgame fortress handling by Lyudmil Tsvetkov, CCC, March 25, 2015
 * A very subtle fortress destruction by Árpád Rusz, CCC, January 23, 2016
 * Fortresses by Jon Fredrik Åsvang, CCC, February 15, 2016
 * Fortresses by Jon Fredrik Åsvang, CCC, January 30, 2017
 * What does semi-fortress mean? by Jon Fredrik Åsvang, CCC, September 22, 2017
 * replace the evaluation by playing against yourself by Uri Blass, CCC, January 25, 2018 » Evaluation
 * evaluation and scaling by Uri Blass, CCC, September 19, 2018
 * Fortress Detection and Evaluation by Stephen Ham, CCC, August 23, 2019

2020 ...

 * Fortress detection? by Martin Fierz, CCC, July 21, 2021 » Checkers
 * Formal definition of fortresses by Hedinn Steingrimsson, CCC, November 10, 2021

=External Links=

On Topic

 * Fortress (chess) from Wikipedia
 * Fortress - chess term | chess24.com
 * A Rare Chess Fortress: Queen vs. Bishop & Knight Endgame • Free Chess Videos • lichess.org
 * Endgame Fortresses • Free Chess Videos • lichess.org by GM Varuzhan Akobian, January 13, 2015

Misc

 * Fortress chess (variant) from Wikipedia
 * Fortification from Wikipedia

=References= Up one Level