Several portions of the wall are actually double, triple, or quadruple walls running closely parallel to one another. This was a common feature in many ancient fortifications because it made the position harder to take. Often the troops would be garrisoned between the walls for protection against surprise attacks from the rear. The land between the walls also offered a protected area for flocks and farmland to provision the troops.
The Chinese team found that the main wall was larger than the others. The investigation continues.
Several walls were originally built starting in the 5th century BC or perhaps earlier. Under the Emperor Qin Shi Huang in c.220 BC, the earlier scattered walls were linked together to make a continuous fortification to protect China from nomadic tribes to the north. The Great Wall was lengthened, added to, and rebuilt several times in later centuries. During the Ming Dynasty (1368–1644) there was a major expansion during which 5,650 km (3,511 miles) of wall were built. A recent survey found the entire wall, with all of its branches, runs for 8,852 km (5,500 miles). This figure will have to be reassessed now that parallel walls have been found.
[Photo courtesy Francisco Diez]