The Great Wall of Ancient China was built by the two dynasties, the Qin first started to building the Great Wall, then the Han continued to build the wall, but the reason why the two dynasties wanted to build the wall was to protect from the Xiongnu and other invaders invading, but all the benefits of the wall didn't outweigh the cost of building wall. The wall was constructed be thousands of soldiers, slaves, peasants, etc. Many died due to to hunger, dehydration, and extreme whether. In Document C, it states, " ... tens of thousands of soldiers died from hunger, sickness, and extreme heat or cold. No exact numbers of deaths are available. "
sacrificing many soldiers to build the wasn't really worth it after a while, even though, there was less people to guard the wall from the Mongols ( Xiongnu ).