The document discusses the history and construction of the Great Wall of China. It describes how the first Great Wall was built during the Qin Dynasty to protect against invaders from the north. Hundreds of thousands died constructing the wall without modern machinery. Subsequent dynasties like the Han and Ming dynasties also built and extended the Great Wall. Today, sections of the Ming Dynasty wall still stand, including the popular Badaling section, which is one of the best preserved parts of the Great Wall.