Silver Yagura.

How it goes together

The king and the left gold stand as in a Gold Yagura, on 8h and 7h, with the left silver on 7g. The right silver stands on 6g, where a Gold Yagura has its second gold, and the right gold stays at home on 4i. The 6g silver guards the 7h gold, and the 7h gold guards both silvers. Both silvers guard the pawns on 7f and 6f. Nothing in the castle guards 5g, beside the 6g silver; in this build only the bishop on 6h does.

Move by move

Black only, from the start, in shogi-gym’s order, the only full one in the sources: the left side goes up as in a Gold Yagura, while the right silver walks 4h, 4g and 5f, waits there while the king goes in, and only then steps back to 6g.

Sixteen moves, four more than our Gold Yagura: the silver takes four steps where the gold takes two, the 4f pawn moves to let it through, and the 5f pawn goes up once it has passed. Its stop on 5f is the Reclining Silver square, and ja.wikipedia says that is where this castle’s silver usually comes from.

What it is good against

The 7h gold. In a Gold Yagura only the king guards it; here the 6g silver does too (ja.wikipedia, shougi.jp). ja.wikipedia also says it is strong against an attack on 7f, which both silvers guard, and shogi-joutatsu that the two silvers and the gold connect well.

Where it breaks

The square beside the silver, and time. A silver cannot step sideways, so the 6g silver does not guard 5g; Shogi Wars says the castle becomes very fragile once the opponent makes a tokin there, and shogijugem names 5g as its weak point. The 7g silver has one defender fewer than in a Gold Yagura, so shogijugem says a knight attack on it is harder to meet. And it costs moves: the right silver, usually an attacker, comes all the way back to defend (ja.wikipedia).

When to build it

Double static rook games, usually as a later stage. ja.wikipedia says it often appears when a Right Fourth File Rook attack on a Yagura turns into a slow game, and that it is often built by pulling a silver back from 5f to 6g; a Bishop Exchange Reclining Silver position can grow into it the same way.