Rather than a single unbroken barrier, China’s legendary Great Wall is actually a collection of stone, wood and earthen barricades that meander for thousands of miles from the Gobi Desert to the North Korean border.
Construction on the fortifications began in the 3rd century B.C. under Emperor Qin Shi Huang, but the most famous sections were erected between the 14th and 17th centuries A.D. to defend the Ming Dynasty against the steppe nomads to the north. These portions stand up to 25 feet tall and were built using bricks and a mortar made from slaked lime and sticky rice. Gates were positioned along key strongpoints and trade routes, and watchtowers were used to send smoke and fire signals in the event of an attack. The completed wall was once the largest manmade object in the world, but despite its grandeur, it often proved ineffective as a defensive barrier. The Mongol leader Altan Khan famously bypassed the wall and raided Beijing in 1550, and the Manchus later broke through in 1644 and brought about the fall of the Ming Dynasty.
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