The Great Wall of China was a series of fortifications built in China in ancient times. It was completed by Qin Shi Huang, the first emperor of unified China, in the 3rd century BC.
At some point between 1968 and 1976, Gary Seven and Roberta Lincoln prevented Chairman Mao Tse-Tung from being assassinated at the Great Wall of China. (TOS novel: Assignment: Eternity)
Captain Kathryn Janeway and Neelix both claimed that it was one of the few man-made objects on Earth which could be seen from space prior to the 22nd century. (VOY episode: "11:59")