China great-wall

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 Chihli of the yellow Sea  
the northern frontier of China to Konsu 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.It is the only structure on earth that can be that can be identified from the moon.