The bolt through the door
In the iron bolt (铁门栓), a center cannon pins Black's middle advisor through his own centre pawn — the advisor dare not move, or the cannon fires at the king. Meanwhile Red's general steps onto the rib, and the chariot slams into the advisor's home corner on the side nobody guards: taking it is illegal (the generals would face), the pinned advisor cannot eat the chariot either, and d9 is covered by the old soldier on c9. Slam the bolt home — the bolt holds because of BOTH mechanisms: the general's glare behind, the pin in front.
Solve the challenge to finish this lesson.