The Great Wall of China — built over the course of two millennia to bolster China's northern frontier — is made of several overlapping walls that run parallel to each other. Collectively, they stretch over 13,000 miles (21,000 kilometers) — more than half the circumference of Earth. But did these extensive walls, with an average height of 25.6 feet (7.8 meters), actually protect China against the outside world?
The answer largely depends on how you define the wall's successes and failures. The Chinese built the wall as a masterpiece of defensive architecture, and while Chinese troops controlling these barriers certainly helped to thwart the attacks of some would-be invaders, the Great Wall was by no means impenetrable. In other words, sometimes it helped protect China, and other times it didn't.
