Did you know the Great Wall of China, one of the Seven Wonders of the Modern World, is the only man-made structure that astronauts can view on Earth from outer space? The Great Wall is not only known for its long history, but its massive construction size, and its unique architectural style. A great army of manpower, composed of soldiers, prisoners, and local people, built the wall. The construction result demonstrates the wisdom and tenacity of the Chinese people. The construction of the Great Wall began between the 7th and 8th centuries B.C. when the warring states built defensive walls to ward off enemies from the north. It was only a regional project then. Until the Qin Dynasty, the separate walls were joined together and it stretched from east to west for about 5000 thousand kilometers and served to keep nomadic tribes out. The Wall was further extended and strengthened in the succeeding dynasties. Especially during the Ming dynasty when the northern nomadic ethnic groups became very powerful, the Ming rulers had the Wall renovated 18 times. The architectural style of the Great wall is a marvel in the history of construction in the world. The weaponry only consisted of swords and spears, lances, bows and arrows. In ancient times, walls with passes, watchtowers, signal towers, together with moats became an important strategy to ensure the safety of the dynasties.