The Great Wall of China is an ancient wall in China . The wall is made of cement, rocks, bricks, and dirt. It was finished in 1878 [1] and it was meant to protect the north of the empire of China from enemy attacks. It is the longest structure humans have ever built. It is about 21,196 kilometers long, 9.1 metres (30 feet) wide and 15 metres high. The earlier sections on the wall are made of compacted dirt and stone. Later in the Ming Dynasty they used bricks. There are 7,000 watch towers, block houses for soldiers and beacons to send smoke signals. Later dynasties adopted different policies towards northern frontier defense. The Han (202 BC – 220 AD), the Northern Qi (550–574), the Jurchen -ruled Jin (1115-1234), and particularly the Ming (1369–1644) were among those that rebuilt, re-manned, and expanded the Walls, although they rarely followed Qin's ...
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