Nakahara.

How it goes together

The king moves one square left, to 6i, and stays on the bottom rank. The left gold goes up to 7h, the right silver to 4h, and the right gold steps sideways into 5i, the king’s starting square: a gold kept on the bottom rank (一段金). The left silver stays at home on 7i. The king guards both golds; the 7i silver also guards the 7h gold, and the 4h silver the 5i gold. 6h, in front of the king, is covered by the king, both golds and the 7i silver.

Move by move

Black only, from the start, in the order of shogi-rule.com’s record, which is also the order of the castle moves in the Japan Shogi Association’s column: the bishop’s pawn, then the left gold, the right silver, the king and the right gold.

Five moves. Once the bishops are traded, as in the Side Pawn Capture, the left silver also goes up to 8h; that is the form White plays with the rook on 8e (△８五飛).

What it is good against

Speed and a low, wide king. ja.wikipedia, after Nakahara, says the formation is low and the king has wide escape routes whichever side is attacked, and that with a gold on the bottom rank it is strong against rook drops and knight attacks. It takes few moves, which matters in Double Wing and Side Pawn games, where the fighting starts early, and shogi-rule.com adds that the king is less likely to end up on the opponent’s bishop diagonal. The JSA column points out that the 4h silver stays in defence, so the attack comes from the knight or a rook swing instead.

Where it breaks

Thickness. Nakahara himself said it is less elastic than the Central House (ja.wikipedia), shogi-rule.com says it does not suit a slow game, and shougi.jp says the pieces are not packed around the king and the head is open to pawns and silvers pushing from above.

When to build it

Double static rook games where the fight starts early: the Double Wing Attack, where Nakahara first played it as Black, and above all the Side Pawn Capture, where White pairs it with the rook on 8e (ja.wikipedia; Shogi Wars calls it White’s standard Side Pawn castle). Nakahara brought it into modern play in the early 1990s and received the Masuda Award for it; ja.wikipedia adds that the shape itself is old and appears in Edo-period books. Watanabe Akira built it as White in game 7 of the 2004 Ryūō match, the game that won him the title (JSA).