Final answer:
The first emperor of the Qin Dynasty, Qin Shi Huang, started the construction of the Great Wall of China in the third century BCE. The wall was built to defend against invasions by nomadic tribes.
Step-by-step explanation:
The Great Wall of China was started by the first emperor of the Qin Dynasty, Qin Shi Huang, during the third century BCE. The construction of the wall was part of a larger effort to defend the Chinese Empire from invasions by nomadic tribes, particularly the Xiongnu from the north. The wall was continuously expanded and rebuilt over the centuries by subsequent dynasties, including the Han dynasty, Ming dynasty, and Qing dynasty.