How did the Qin dynasty connect earlier defensive walls into the Great Wall system?

Following the unification of China in 221 BCE, the first Qin emperor, Shihuangdi, sought to consolidate his newly formed empire. To facilitate internal administration and movement, he ordered the removal of existing fortifications that had previously separated the warring states.

Simultaneously, he tasked General Meng Tian with securing the northern frontier against the Xiongnu nomadic tribes. To achieve this, Meng Tian linked the disparate defensive wall segments previously constructed by the states of Qin, Yan, and Zhao. This massive project, which began around 214 BCE and lasted for a decade, resulted in the creation of the "10,000-Li Long Wall." This system utilized hundreds of thousands of soldiers and conscripted laborers to integrate the earlier regional barriers into a unified northern defensive line.1