The Great Wall Explained: 2,000 Years of the Largest Defense Project in History

Six dynasties spent 2,000 years building it. A million people died on it. It runs for 13,170 miles. And the question almost no one asks is whether the Great Wall of China actually worked. How did six different Chinese dynasties — each facing a different enemy, each using different building techniques — keep returning to the same defensive solution for two millennia?