◇ Just read — no move neededFiring from behind the wall
The 沉底炮 — the sunk cannon — works from the last rank, where almost nothing can reach it. Its power is the power of every cannon, turned inside out: it needs exactly one screen, and on the back rank the defender's own advisors and elephants volunteer for the job. The advisor in front of the king, the elephant on the corner — each one is a screen the cannon did not have to build. And because the bottom rank is so quiet, the cannon often sits there for free.