Complete Yagura.

How it goes together

Start from a Gold Yagura: king on 8h, golds on 7h and 6g, silver on 7g. Add the right silver on 5g, beside the 6g gold, with the 5f pawn in front of it. The two silvers now stand on one rank, 7g and 5g, with the gold between them. The 6g gold guards the 5g silver, and the 6f pawn in front of that gold has three defenders: both silvers and the gold. Its other name is 四枚矢倉, the four-general Yagura.

A Complete Yagura is a Yagura with three more moves: the first twelve below are that castle.

Move by move

Black only. The first twelve moves are our Gold Yagura; then the fifth-file pawn goes up to free 5g, and the right silver climbs 4h, 5g.

Fifteen moves in all. The bishop is still on 6h, and the two silvers now block both of its forward diagonals, so it can only step back to 7i or 5i. ja.wikipedia says players usually build this castle once the bishop has gone out to 4f.

What it is good against

Thickness. Four generals around the king make it very hard to break from above, which is the first thing Shogi Wars and shogi-joutatsu say about it. It also costs little to reach: three moves on top of a Gold Yagura.

Where it breaks

Attack. The right silver is the piece a Yagura player usually attacks with, and here it defends, so the attack runs thin. The two silvers on one rank give the opponent a target: after a pawn exchange on 6e, a knight that recaptures there attacks 7g and 5g at once (en.wikipedia).

When to build it

Double Yagura games, more often by White (ja.wikipedia). When both players build it, the long-held view was that neither side can start a fight without getting the worse of it, so the game tends to end in a repetition draw (千日手); Yonenaga Kunio and Tanigawa Kōji were among the players who looked for ways to break that.