Millennium.

How it goes together

Named for the years around 2000, when professionals started playing it on purpose. It begins like a boat castle. Then the bishop steps out to 6f and the knight jumps to 7g, which empties 8i; the silver comes across to 8h, the king goes down to 8i, and the golds fold round him onto 7h and 7i. ja.wikipedia records its other names, among them 三浦囲い after Hiroyuki Miura, who established it in professional play (he credits Osamu Nakamura with playing it first), and トーチカ for this form with the bishop on 6f.

Move by move

Black only, from the start position. A boat castle’s first moves, then the bishop and the knight step out of the way, and the king goes down behind them.

Thirteen moves, in the source’s order. The knight on 7g did the first job by moving: it made room on 8i. It now stands on the head of the 7h gold, and ja.wikipedia counts that knight’s freedom to join an attack among the castle’s modern uses.

What it is good against

Hardness close to an Anaguma, in the source’s words, with the king one file in from the edge. It was developed against the Fujii System, the ranging rook plan built to break an Anaguma early.

Where it breaks

Time, and range. Thirteen moves is a long build, and the source’s second complaint is that it is a castle for one opponent: made against the Fujii System, it had few other games to be used in. ja.wikipedia adds that since about 2018 it has come back, now as a castle whose left knight can join an attack, played without the Fujii System in view.

When to build it

A static rook against a Fourth File Rook, above all when the Fujii System is on the board or likely; and, in its modern form, whenever you want the left knight free to attack.