Anaguma.

How it goes together

The lance steps up from 1i to 1h and the king takes its corner. The silver closes the hole on 2h, one gold stands on 3h and the other on 3i, and the knight never leaves 2i. Few pieces can reach a king in the corner without first breaking through three generals and a lance; the knight, which jumps, is the exception.

Move by move

Black only, from the start position. The first four moves are a Mino, and then the castle does the thing no other castle does: the lance steps up, and the king drops backward into the corner it has opened.

Eleven moves, and the last three are one gold walking across, 6i to 3h. Not one move in that list threatens anything, and that is precisely what the opponent is being handed in exchange.

What it is good against

No castle is harder to finish off. The king is not beyond check: once the 2g pawn has moved, a knight dropped on 2g checks it and hits the 3i gold at the same time. But while the three generals stand around it, a check is rarely the start of a mate, and that is what lets you attack with everything else. The common Anaguma plan is to trade pieces until the opponent’s thinner castle gives way first. Many top-level games are decided by who gets to build one.

Where it breaks

It is slow, it gives up the corner as an escape (the king has nowhere left to run once the wall goes), and while it is being built the pieces that will form it are not doing anything else. An opponent who attacks early, before the hole is sealed, is the Anaguma player’s worry.

When to build it

When you expect a long game and the opponent is not attacking yet. In the ranging rook form shown here the rook has gone left; static rook players build the same castle in the left corner.