The Great Wall of China is the largest defense fortification and the greatest building enterprise ever undertaken by man. About 1500 miles long, this unique wall stretches from the Gulf of Chili of the Yellow 9০৪ along the northern frontier of China to Kans Province in the west. The width of this wall varies from 15 to 40 feet at the base, and from 12 to 35 feet at the summit. Its height ranges from 20 to 50 feet. This is the only structure on the earth that can be identified from the moon. Walled frontier between the kingdoms in China date from at least the 4th century BC. in the 3rd century BC, the first emperor of 'Ch 'in', after uniting China, linked up the existing walls and built new sections to create the Great Wall as a continuous unit. His purpose was to defend China from the Huns on the north. Watch towers and garrison stations were set at regular intervals in order to Send signals and guard the bulwark. The wall was constructed in such a way that troops summoned by beacon fire could be quickly 10 transported along its top. The wall was built mostly of rubble, although the western sections are of tamped earth and important passes are of brick or masonry. The wall was frequently modified in later centuries. Some of its sections were built in the 15th and 16th centuries with new facilities of high towers. Before that, extensive repairs and extensions were made at various periods, notably during the Ming dynasty.