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The Great Wall of China is one of the wonders of the modern world and became a UNESCO Heritage Site in 1987. It is also one of the longest structures in the world, stretching for 6,700 km, and has a history of more than ___two thousand years. The building of the wall began between the 7th and 8th centuries BC as a means of defending most of a China from invading people from the north. Rulers of different parts of the country built sections of the wall, which were later joined together during the time of the Qin dynasty. During the Ming dynasty (1368-1644 AD), the wall was repaired, extended, and took on the appearance it has today, with a complex system of forts and towers. It has an average height of ten meters and a width of five meters, and it runs from east to west.