What is a "sentry"?

In the Han Dynasty, there was a form of professional soldiers, that is, "Ding Tian Bing". In the Han dynasty, it was very expensive to defend the northern border defense line stretching for thousands of miles, and it was also very expensive to supplement the border defense forces only by the rotation of garrison soldiers. Chao Cuo, the adviser of Emperor Wen of Han Dynasty, suggested "emigrating to the frontier" and began to organize emigrating to the frontier. Later, Emperor Wu of the Han Dynasty further reclaimed land and moved the recruited soldiers to the frontier, where they lived, farmed and fought. These soldiers became special professional soldiers. They got a piece of wasteland from the country and some subsidies, but they usually live on their own farms and take turns to be on duty. This military system later became the traditional policy of the Han Dynasty, which was used until the Eastern Han Dynasty.

Terracotta Warriors and Horses of the Qin Dynasty