Why did Qin dynasty build a long defensive wall?

The Qin dynasty built a long defensive wall to protect its territories from invasions by nomadic tribes from the north, particularly the Xiongnu. This wall served as a crucial military defense, helping to consolidate the newly unified Chinese state under Qin Shi Huang. Additionally, it aimed to assert control over border regions and facilitate trade and communication across the empire. The construction of the wall also represented the strength and ambition of the Qin dynasty in maintaining stability and security.