<human>:  When was the Great Wall of China built?
<bot>: The Great Wall of China was built by Emperor Qin Shi Huang in the third century BC. It was a massive defensive structure built to protect the Chinese empire from the nomadic peoples of Central Asia. The wall was made up of earth, stone, and brick, and was about 5,500 miles long. It was also about 1,300 feet high, with watchtowers every 100 miles.