Fortress castle

Gold Fortress
☖ pieces in hand: –
987654321 
         1
         2
         3
         4
         5
  歩歩歩    6
歩歩銀金     7
 玉金      8
香桂       9
☗ pieces in hand: –

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.[1]

A yagura

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.

History[edit]

Hon'inbō vs Ōhashi 1618
position after 50 moves
☖ pieces in hand: –
987654321 
香桂  角  桂香1
 王金   飛  2
   金 銀 歩 3
歩銀歩歩歩歩  歩4
 歩    歩歩 5
歩 歩歩歩歩  歩6
 歩銀金 銀   7
 玉金   飛  8
香桂角    桂香9
☗ pieces in hand: –

A Fortress castle structure was first formed by Sansa Hon'inbō in 1618 who was playing a Static Rook position (eventually Sleeve Rook) against Sōkei Ōhashi I [ja] who was playing a Fourth File Rook position (eventually Third File Rook).[2][3]

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.

The basic form[edit]

Basic structure
☖ pieces in hand: –
987654321 
         1
         2
         3
         4
         5
  歩      6
歩歩銀      7
  金      8
香桂       9
☗ pieces in hand: –

The Fortress castle has the left silver on the 77 square and the right gold on the 78 square. In particular, the silver on 77 is the quintessential characteristic that defines the Fortress. Most Fortress castle variants have this structure. The seventh file pawn must be pushed in order to make way for the silver.

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.

Simple Fortress
☖ pieces in hand: –
987654321 
         1
 飛       2
  桂   角歩 3
  歩   歩  4
       歩 5
  歩 歩 歩  6
歩歩銀歩     7
  金角   飛 8
香桂       9
☗ pieces in hand: –

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.

Gold Fortress[edit]

A common Fortress structure is the Gold Fortress (金矢倉 kin yagura), and usually when the term Fortress/矢倉 is used (especially in the Yagura opening), it implies that the variant referred to is the Gold Fortress. It has a strongly protected king and a well-fortified line of pawns. It is difficult to break down with a frontal assault, though it is weaker from the side. It is typically used against Static Rook openings that involve advancing the rook's pawn. However, one's opponent may just as easily adopt this defense, giving neither side an obvious advantage.

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.[1]

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.[1]

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.

Classic Fortress development (early S-77)[edit]

The development of the Gold Fortress castle in the Classic Fortress (旧矢倉) opening is shown below for Black. This is a 13-move sequence that is fairly typical.[4][5]

Classic Fortress
☖ pieces in hand: –
987654321 
         1
         2
         3
         4
         5
  歩歩歩    6
歩歩銀金     7
 玉金角     8
香桂      9
☗ pieces in hand: –
(xiii) K-88

New Fortress development (early P-66)[edit]

The development of the Gold Fortress castle in the New Fortress (新矢倉) opening for Black is shown below.[6] The New Fortress castle construction differs from the Classic Fortress construction starting from the player's third move.

New Fortress
☖ pieces in hand: –
987654321 
         1
         2
         3
         4
         5
  歩歩歩    6
歩歩銀金     7
 玉金角     8
香桂      9
☗ pieces in hand: –
(xiii) K-88

Quick Fortress development[edit]