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.