<human>:  When was the Great Barrier built?
<bot>: The Great Wall of China was built between 220 and 210 BC. It was a defensive structure built to protect the Chinese Empire from invasion by nomadic tribes. The Great Wall of China is the longest man-made structure in the world. It is approximately 8,851 km (5,500 mi) long, and varies in height from 2 to 15 meters (6.6 to 49 ft).