The bolt through the door
The iron bolt (铁门栓) mates through a pinned door. The central cannon stares down the e-file, and because Black's own centre pawn stands at e7, the advisor on e8 is frozen: one step aside and the cannon aims straight at the king. That pin is the licence for the punch — with Red's general glaring up the f-file from f0, the chariot rides into f9 untouched: the king cannot take it (the two generals would face), the pinned advisor dare not move, and the old soldier on c9 bars d9. Pin behind, glare in front — the door is bolted from both sides.
Solve the challenge to finish this lesson.