Ming Dynasty rulers, plagued by raids by Mongolian and Turkic tribes, beginning in the 14th century strengthened and extended walls built centuries earlier. The 300-year project produced the longest structure ever built — nearly 4,000 miles of fixed-stone fortification across north-central China. The Great Wall proved of limited military value. Mongol leader Altan Khan raided Beijing, hastening the fall of the Ming Dynasty, before the wall was finished. It's now one of the world's great tourist attractions.
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