Fortress opening

Fortress (矢倉 or 櫓 "yagura") is both a Static Rook opening (矢倉戦法 "yagura senpō") and a castle in shogi.
It is usually played in a Double Static Rook opening, which is often a Double Fortress opening. However, it may also occur in different Double Static Rook openings such as Fortress vs Right Fourth File Rook.
The Fortress castle (矢倉囲い "yagura gakoi"), which is the defining characteristic of Fortress games, was considered by many to be one of the strongest defensive positions in Double Static Rook games in the 1980s.
The term "yagura" is the Japanese word for a tower-like structure in traditional Japanese castles.
Double Fortress.
The most commonly encountered Fortress strategies occur in Double Fortress games where both players use a Fortress formation.
Historical Fortress.
Earlier josekis for Fortress in the Edo period (usually spelled 櫓 at that time) were very different from the current josekis.
For instance, in one variation, it is White who delays pushing their rook pawn (whereas Black delays the rook pawn push in the modern era) and an early bishop trade occurs before both players' kings are moved into their castles.
vs Snowroof.
The Snowroof strategy can be played against a Fortress opponent.
Rapid Attack Fortress.
Rapid Attack Fortress strategies are characterized by the fact that White will start the fight following Black's blocking of the bishop's diagonal. These are active plans where White wants to avoid a development in which Black takes hold of the initiative. The side playing Fortress aims at building up a position wary of the rival's rapid attack. There are a number of variations:
The rapid attack strategy will vary depending on whether in the fifth move Sente plays P-66 or S-77. For example, both Sitting King Bogin and Right Fourth File Rook are played following P-66, while Central Rook Fortress and Akutsu Rapid Attack Fortress are played following S-77. Yonenaga Rapid Attack Fortress can be played against either.
vs White's Left Mino.
1. P-76 P-84
2. S-68 P-34
3. P-66 S-62
4. P-56 P-54
5. S-48 G61-52
6. G49-58 S-32
7. G-67 P-44
8. S-77 G-43
9. B-79 B-31
10. P-36