突門防禦
Breakthrough Gate Defense
城百步一突門，突門各為窯灶，竇入門四五尺，為其門上瓦屋，毋令水潦能入門中。吏主塞突門，用車兩輪，以木束之，塗其上，維置突門內，使度門廣狹，令之入門中四五尺。置窯灶，門旁為橐，充灶伏柴艾，寇即入，下輪而塞之，鼓橐而熏之。
Every hundred paces of wall, construct a sally port. At each sally port, build a kiln-oven with a flue extending four to five chi into the gate. Build a tiled roof over the gate to prevent rainwater from entering. Assign an officer to block the sally port. Use a two-wheeled cart bound with timber, its top plastered, placed inside the sally port, sized to fit the gate's width, extending four to five chi into the passage. Place kiln-ovens and bellows beside the gate, with mugwort and brushwood piled in the ovens. When the enemy enters, lower the wheels to seal the passage, and pump the bellows to fill it with smoke.
Notes
The 'breakthrough' (突) refers to enemy forces breaking through a gate or wall breach. The Mohist counter-measure is an ingenious smoke trap: allowing the enemy into a confined sally port, then sealing it and pumping smoke and heat from ovens to neutralize the attackers.