A pile of bricks
In the 7th century BC minor walls were built to protect the vassal states. These were linked together during the Qin Dynasty in 200 BC, forming the original Great Wall to keep out the marauding nomads in the north. Most of the remaining sections seen today were largely rebuilt during the Ming Dynasty 1300-1600 AD. It stretches across the land for 2700 km and the peak stretch is estimated to have been 6700 km. But no, it cannot be seen from space.
Great Wall, Badaling section