Treatise on Armament Technology, Volume 39

[The city] was expected to surrender. The rebels pulled back their army to wait for it. [Li] Baoyu, however, repaired the city defenses, and the next day he requested to engage in battle once more. He deployed a "surprise force" qi bing; a tactical unit used for unexpected maneuvers or ambushes to strike from both sides, killing and wounding a great many of the enemy.

At that time, [Li] Guangbi was stationed at Zhongtan City, where he had erected wooden palisades. Outside the palisades, he had trenches dug. The rebel general Zhou Zhi attacked the position. Guangbi ordered Li Fei Yuanli, the Military Governor Jiedushi; a powerful regional military commander of the Western Garrison, to lead elite troops out from the "sheep-and-horse wall" yangma cheng; a secondary outer defensive wall used to protect livestock and act as a buffer zone to resist the rebels.

The rebels filled the trenches in eight places and cleared the terrain to create paths for their advance. Guangbi sent a messenger to ask [Li Fei] Yuanli: "The Vice Censor-in-Chief Zhongcheng; referring here to Li Fei Yuanli's court rank watches the rebels fill the trenches and approach our palisades, yet he remains calm and motionless. Why is this?"

Yuanli replied: "Does the Minister of Works Sikong; an honorary high office held by Li Guangbi wish to defend, or to fight?"

Guangbi said: "I wish to fight."

Yuanli said: "If you wish to fight, then why should we stop the rebels from filling the trenches? Yuanli implies that the rebels are unwittingly clearing the way for a Tang counter-charge."

Guangbi said: "Excellent. I had not thought of that. Carry on!"

Yuanli waited for the gates in the palisade to open, then led a "death-defying squad" gan si shi; elite shock troops prepared for high-risk missions to charge out suddenly. They struck with such ferocity that they broke the enemy's lines. Zhou Zhi gathered his remaining troops and hurried toward the Northern City. Guangbi climbed the city wall to observe the battlefield. He said: "The rebel army..."