Big Four.

How it goes together

Start from the Static Rook Anaguma: king 9i behind the lance on 9h, a silver on 8h, golds on 7h and 7i, and the right silver still out on 5g. The right silver walks home by 6f to 7g, after the bishop steps back to 6h to make room. Then the 8g pawn steps up to 8f, the 8h silver climbs to 8g, and the 7i gold takes its place on 8h. The result is a block of four: golds 8h and 7h side by side, guarding each other, and silvers 8g and 7g in front of them. Each gold guards the silver in front of it, and each silver guards the gold diagonally behind it.

The Big Four is a Static Rook Anaguma with six more moves: the first fifteen below are that castle.

Move by move

Black only, from the start position: the Static Rook Anaguma’s fifteen moves, then six more in the order shogi-gym gives. The Japan Shogi Association’s column reaches the same squares through the Silver Crown Anaguma instead.

Twenty-one moves. The right silver has come all the way home, the left silver has stepped up, and the lid on 8h is a gold now. What is left for the attack is the rook, the bishop and the knights.

What it is good against

It is very hard to break. shogi-gym says it is strong from above and at the edge as well as from the side, and that it stands up to a tokin attack because the gold beside the king is one rank up; shogi-rule.com says it has almost no weak point as a castle. The JSA column says some call it the strongest castle ever.

Where it breaks

Time and attackers. It takes twenty-one moves, six more than the Anaguma it grows from, so a rapid attack can catch it half-built. With four generals in defence, the attack has to be carried by the rook, the bishop and the knights, and Shogi Wars warns that this leaves few pieces to attack with.

When to build it

A static rook against a ranging rook in a slow game, when the opponent gives you time to keep adding generals (shogi-gym). ja.wikipedia says it is easiest to reach against a Third File Rook, where the right silver can travel by 6f to 7g. A ranging rook can build the mirror image too: ja.wikipedia shows the Central Rook side of a double Anaguma game building one.