The Great Wall of China is a series of fortifications made of stone, brick, tamped earth, wood, and other materials, generally built along an east-to-west line across the historical northern borders of China in part to protect the Chinese Empire or its prototypical states against intrusions by various nomadic groups or military incursions by various warlike peoples or forces. Several walls were being built as early as the 7th century BC;these, later joined together and made bigger and stronger, are now collectively referred to as the Great Wall.
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I took an overnight train from Nanjing and reached Beijing at 6 AM