Why did shihuangdi order farmers to strengthen walls along the northern border?

Shihuangdi ordered farmers to strengthen the walls along the northern border to protect against invasions from nomadic tribes, particularly the Xiongnu. These fortifications were part of his broader strategy to consolidate and secure the newly unified Chinese state. By involving farmers in this effort, he also aimed to boost agricultural productivity and ensure the stability of his empire. Strengthening the walls was a crucial step in safeguarding the territory and maintaining control over the region.