Was the Great Wall of China really built to keep out invaders?

The answer

While the Great Wall's primary stated purpose was military defense against northern nomadic groups like the Xiongnu and Mongols, it actually served more effectively as a border control and customs checkpoint - regulating trade along the Silk Road and collecting tariffs. The wall as we know it today was mostly built during the Ming Dynasty (1368–1644) and stretches approximately 21,000 kilometers including all branches. Despite its scale, it was breached multiple times, most notably when the Mongols simply bribed a general to open a gate in 1644.