Flatfish.

How it goes together

Swing the rook to 5h, then walk the king to 2h and lift the right silver to 3h, as in a Half Mino. Then the left gold steps sideways to 5i, under the rook, and the left silver goes up to 6h. The two golds now stand side by side on 5i and 4i and guard each other. The 4i gold and the 3h silver guard each other, so do the 5i gold and the 6h silver, and both golds guard the rook. 平目 is also the name of the Central Rook strategy built around the castle, and ja.wikipedia files the castle as a form of the Mino.

Move by move

Black only, from the start position. The rook goes first, as ja.wikipedia says; then the king and the right silver along the Half Mino’s route, then the left gold and the left silver. ja.wikipedia lists the silver before the king, which also works if the king goes round by 3i. The order of the last four moves, three pawns and the bishop, is ours; they reach the article’s diagram.

Eleven moves, ending with the bishop on 7g, where it can be traded. ja.wikipedia’s plan from here is to trade bishops, jump the knight to 7g and exchange the central pawn.

What it is good against

Solidity. All four generals are in the castle, and ja.wikipedia says they connect well, sit low and hold up against a dropped rook or bishop. That is why a Flatfish player can trade bishops on purpose.

Where it breaks

The left side. With all four generals on the right, the left keeps only its knight and lance, and on the bottom rank 7i is guarded by the 6h silver alone. Against a bishop that promotes on that side, or a break on the eighth file, the theory ja.wikipedia gives is to give up the left lance.

When to build it

A Central Rook strategy that uses this castle in place of the usual Half Mino. ja.wikipedia adds that the stronger player uses it in lance-handicap games. Shogi Wars says it is the two golds side by side that look like the fish.