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Fortress or Yagura (矢倉囲い or 櫓囲い yaguragakoi) is a castle used in shogi. It is considered by many to be the strongest defensive position in shogi in Double Static Rook games.
The most common form of Fortress is the Gold Fortress. Besides the Gold Fortress, there are many variant forms. And, there are development relations between Fortress and other castles. For instance, the Crab castle can be developed into a Gold Fortress, and the Gold Fortress can be developed into a Fortress Anaguma.
Fortress is also the name of an opening strategy that uses a Fortress castle (see: Fortress opening). When used in the Fortress opening, the bishop, rook, and a pawn all support a later attack by the rook's silver or knight. Variants of the Fortress are also used in other Double Static Rook games (for instance, Bishop Exchange) as well as in Static Rook vs Ranging Rook games and in Double Ranging Rook games.
In the modern era, Fortress castles are typically used in Static Rook positions against Static Rook opponents (that is, in Double Static Rook games). However, Static Rook positions playing against some Bishop-Exchange Ranging Rook position may also use a Fortress variant after the bishop trade off. Furthermore, Fortress castles constructed on the right side of the board may be used in Double Ranging Rook games.
When the king is moved into the castle, then it will be positioned on the 88 square although there are exceptions where the king remains outside the castle (on 68 or 69 or 79) or on a different square in some variants (such as on 78).
This basic form can occur in games when the player attacks early without extensive defensive preparations. For instance, the basic shape is usually formed in Bishop Exchange openings and this may be the only castle development in the faster attacking strategies such as Climbing Silver or Rushing Silver.
When used in the Fortress opening, a simple Fortress can be constructed with the S-77 G-78 formation and with the fifth file pawn typically pushed so that the bishop can be utilized early for counterattacking purposes (such as trading off the third file pawn, or exchanging the bishops off the board via P-24 Px24 Bx24 Bx24 Rx24 if the opponent's bishop is on 33, or by moving to 46 to attack the opponent's rook on 82 or to attack the opponent's jumped knight on 73 possibly with supporting a P*74 pawn drop or P-74 pawn push). This shape of Black's shown in the adjacent digram is actually found as transitional development in the Classic Fortress variation of the Fortress opening.
The sixth pawn file is often pushed to 66 although this is not always the case (for instance it is not always pushed in the Helmet Fortress in Bishop Exchange openings and in other cases it remains on 67 so that the left silver can move to 66).
Although these can be thought as the basic structure of the Fortress in the abstract, the most common form of the Fortress is the Gold Fortress.
There is a good deal of flexibility in the order of moves when building the Fortress defense. A point to keep in mind is that the generals should move diagonally, not directly forward.
While forming the castle, the rook's pawn is often advanced two squares in preparation for a rook assault on the opposing king. Another common preparation is to advance the adjacent silver's pawn square, allowing passage for both the rook's silver and knight. These offensive moves are not properly part of the castle, but the two-square pawn advance must be carried out early if there is to be room for it, and so it is often done while still castling.
In the following three sections are possible typical developments of a Fortress castle in Double Fortress openings. The first are normal castle development sequences of nine moves. The last section shows the development of a Quick Fortress which is one move faster than usual methods.
However, the specific order of the castle move sequence will vary depending on White's piece development. Moreover, the castle usually isn't built all at once – rather, castle development is interwoven with the development of the attacking pieces. Additionally, moving the king all the way into the castle to the 88 square is not always optimal depending on the state and nature of White's attack.
Quick Fortress castling will lead to the intermediate position seen in the adjacent diagram. This intermediate position does not occur when a Fortress is built using the slower traditional move sequence.
(The term 早囲い hayagakoi also refers to the Quick castle, which is otherwise known as the Wall castle.)
The Tofu Fortress develops from a Gold Fortress when the opponent attacks the Fortress's silver on 77 with a knight jump from 73 to 85. Since the silver–knight exchange is not favorable to Black, the silver escapes the attack by moving to 86. Unfortunately, this removes one piece that is closing the diagonal that the opponent's bishop is attacking. The second attacking move is the ...P-65 pawn. Black cannot capture this sixth file pawn with Px65 since their sixth file pawn on 66 is pinned by White's bishop. However, there are no other satisfactory responses. If Black does not capture the pawn, then ...Px66 attacks Black's right gold on 67. Moving the gold leftward with G-77 places the gold into White's knight attack. Moving the gold rightward with G-57 will be followed by ...Px66 which is threatening the P-67+ that will attack both of Black's golds and bishop and simultaneously give a discovered check on Black's king allowing White to capture either gold after Black resolves the check. Moving the king off the diagonal with K-79 still leaves an unfavorable state where ...Px66 will attack Black's gold again threatening P-67+ with a similar set of attacked pieces except now with a discovered attack on Black's left lance by White's bishop.
This type of Anti-Fortress attack occurs in Counter-Fortress Rapid Attack strategies, Left Mino Right Fourth File Rook, and some Snowroof positions.
The 1976 May 13 Meijin game between Kunio Yonenaga and Makoto Nakahara is an example of such a game.
The king can be moved fully into the castle on the 88 square but often it remains outside the castle on 79 or 68 (or more rarely 69).
The right gold is typically not moved up to 67 but remains on 58 so that it can defend more widely against the opponent's bishop drops. There is even a variant in which the right gold is positioned on 48 so that it defends an even wider area.
The edge pawn is often pushed to P-96 especially in the Reclining Silver variations.
As a result of the right gold remaining on rank 8, the sixth file pawn is often not pushed to 66. In the Reclining Silver variations of Bishop Exchange, some subvariations do move the 66 so that it may capture the opponent's pawn on that file (with ...P-65 Px65). However, in some subvariations, the P-66 pawn may not be considered advantageous.
A Helmet Fortress can be developed into a Dented Fortress by moving the right gold to 68.
In a Reclining Silver variation with P-66, the reclined right silver on 56 can possibly be moved to 67 to develop into a Silver Fortress.
The Dented Fortress is often in Bishop Exchange openings as a development from the Helmet Yagura castle.
It is typically used by White in the Double Reclining Silver variations of that opening.
The sixth file (or fourth file for White) pawn can also be pushed so that this castle can be developed into a Gold Fortress.
This variant is not as strong as the Gold Fortress; however, it does protect more widely within the player's camp which can be important if the opponent's bishop if off the board. (Note is this the same reasoning used for the Incomplete Fortress where the left gold is more central on 68 instead of 78 and the Helmet Fortress used in Bishop Exchange games with its right gold on 58 or 68.)
The castle was recently prominently used by Taichi Takami in a 2018 Eiō title tournament.
The High Silver Fortress is used in the King's Head Vanguard Pawn strategy, which is a Counter-Ranging Rook position.
The Silver Square Fortress is structurally similar to the Silver Fortress (silvers on 77 and 67), the Incomplete Fortress (gold on 68), and the Big Four castle (which also has this same square silver-silver and gold-gold arrangement). This castle is very strong. Firstly, a bishop is unable to drop to the yellow squares which are possible with a Gold Fortress. Secondly, 57 is protected by the 68 gold. Thirdly, the 76 and 66 squares are not weak points. But, it is dangerous if attacked on the left edge.
The other related form is the Gold Square Fortress. This uses the same 2x2 four general square shape but has the typical gold on the 67 square as in the Gold Fortress. A silver is placed on 68 to complete the square.
A Right Fortress may be useful in Double Ranging Rook positions since the Fortress structure is strong against attacks from above.
A demerit of Right Fortress is that it requires several moves to create (compared to other Ranging Rook castles like Mino castle).
A Mino castle may transition into a Right Fortress.
The crab name comes from the way the king at 69 can only move from side to side (79, 59).
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