Fortress castle

It is considered by many to be the strongest defensive position in shogi in Double Static Rook games.

The earliest recorded use of a Fortress castle structure was by Sansa Hon'inbō in a game against Sōkei Ōhashi I [ja] in 1618.

It is plausible that Sansa and Sōkei invented the structure in earlier unrecorded games played against each other, although this is not confirmed.

Furthermore, Fortress castles constructed on the right side of the board may be used in Double Ranging Rook games.

This basic form can occur in games when the player attacks early without extensive defensive preparations.

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.

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.

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.

A common attack against the Fortress defense is to advance the rook's knight directly forward, defended by the rook and with a pawn in hand, to attack the fortifications on either side of the castled king.

Many of the variants are designed to address weakness in the Gold Fortress's structure under certain situations.

Since a silver–knight exchange is often not favorable, the silver remains on 68 where it is safe from attack and also defends the 57 square (in case of a knight to 65).

This form also allows the left knight to capture/recapture on 77 as well as close the long diagonal that the opponent's bishop may be attacking on.

The joseki for the Double Complete Fortress can often lead to a repetition draw since both sides are felt to be in a zugzwang-like situation.

Nonetheless, it is still possible to play a Double Complete Fortress game without a repetition draw.

Both players first built a Helmet castle as is typical of Bishop Exchange games.

This gave Watanabe (Black) the time to develop his Helmet castle into a Silver Fortress by pushing his sixth file pawn (P-66), moving his reclined silver from 56 to 66, moving his king into the castle (K-79, K-88), and pushing the central pawn (P-56).

The Helmet Fortress (兜矢倉 kabuto yagura) or simply Helmet or Headpiece (カブト kabuto) castle is a variant used in Bishop Exchange games especially in Reclining Silver variations where there is more defensive development before the players start their attacks.

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.

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).

[18] 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.

The castle was recently prominently used by Taichi Takami in a 2018 Eiō title tournament.

Firstly, a bishop is unable to drop to the yellow squares which are possible with a Gold Fortress.