Let me tell you a story for why people built The Great Wall.
We thought that The Great Wall was built by Chinese for defense horde, which is true for the Ming Great Wall, which is The Great Wall in Badaling, near to Beijing. When you visit it, you will always hear something like “The Great Wall is more than 2500 years old” However, that is not true for the Ming Great Wall. When you are talking about the 2500 years old Great Wall, it should be The Great Wall built in Qin. Which has a different story.
There is a reason why Qin people who work agriculture in China built the wall: The Great Wall is more for attack rather than defense.
Here are some evidence:
First, the structure of the wall:
“Three points about the walls’ construction should be emphasized. First, 40 percent of the total line of the wall was built on sloping terrain: the higher ground was kept on the inside, a moat was dug on the external (and lower) side, and soil and stones were piled up into a wall. Second,fortifications have been found near ravines, crevices, and rivers encased in steep gullies. We do not find actual walls made of stamped earth, but ele- vated platforms and occasionally a fort blocking the entrance to a valley. This type of system accounts for 20 percent of the total length. Third, walls made of stamped earth are seen across plateaus and flat or mildly rolling plains. For construction material the builders used what was available at a location – earth, stone, or a combination of both.“^1
The structure makes the walls in Spring and autumn period seem more for attack rather than defense. It stores war tools, place for solder to rest.
Second, three states Yan, Qin, Zhao occupied land in north which was land of XiongNu(142)
1.Nicola Di Cosmo, Ancient China and Its Enemies : The Rise of Nomadic Power in East Asian History, (Cambridge, UK: Cambridge University Press, 2002.),146