<human>:  When was the Great Wall of China built?
<bot>: The Great Wall of China was built in the 5th century BC. It was built as a defensive wall to protect China from the nomadic peoples to the north. It was built as a series of walls, forts, and watchtowers along the northern border of China. The wall was constructed by the Qin dynasty, who ruled from 221 BC to 206 BC.