◇ Just read — no move neededDoors and doorstops
The two advisors are the palace doors: each one covers the square the king would flee to, and each one blocks the cannon's view — until it falls. When an advisor is captured, its square becomes a landing pad: a cannon parked on a fallen door square cannot be taken (the king would be stepping into the eyes of whatever guards it), cannot be blockaded, and stares straight at the king's remaining door. The defense, 补将门, means never letting both doors fall — and when one is gone, knowing that the last advisor is now the whole wall.