Explain the rationale behind the building of the Great Wall of China

What will be an ideal response?

During the Qin dynasty, the most serious challenge to Qin safety came from the repeated invasions by the nomadic Central Asian peoples along China's northern borders. To discourage invasion, the Qin commissioned the construction of the roughly 3,000-mile-long Great Wall, which stretched from northeast to northwest China. Although the Great Wall of China could not wholly stop an army on foot, it discouraged mounted men, wagons, and the like from making raids across the borders. The building of the Great Wall required the labor of some 700,000 people. Started in the year 214 B.C.E., the final result was not finished until the seventeenth century C.E.