Fortress castle

Fortress or Yagura is a castle used in shogi. It is considered by many to be the strongest defensive position in shogi in Double Static Rook games.
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.