Context From Wiki:

https://en.wikipedia.org/wiki/Battle_of_Ansi

Tang [Chinese] forces attempted to build a rampart in the southeastern corner of the fortress, gradually approaching the wall. Meanwhile, the external wall was constantly razed by the garrison. Li Daozong was injured in the battle. The Tang used soldiers and an unknown number of laborers (possibly war captives) to build the rampart and the top of the rampart came to only a few feet away from the fortress.[5] It overlooked the city. Fu Fuai, one officer of the Tang Army, stationed his troops on the top of the rampart. However, Fu Fuai left the camp privately and the rampart suddenly fell, and the Goguryeo [Korean] army occupied it. Taizong was very angry and put Fu Fuai to death. After that, Tang tried to regain the rampart for three days but failed. On the third day, the Goguryeo army reinforcements arrived and Taizong had still not captured the Ansi Fortress.