The document discusses the history and construction of the Great Wall of China. It was first built in the 7th century BC and rebuilt over successive dynasties to protect against northern invaders. Around 300,000 soldiers and 500,000 civilians helped construct the wall. Most of the current wall was built during the Ming Dynasty in the 15th-16th centuries. It is over 13,000 miles long and a popular tourist attraction, receiving over 41 million visitors annually.