Great Wall of China: Between 8th and 7th centuries B.C. (BCE), the warring states in China built defensive walls to pretect themselves from enemies from the north. During Chin (Qin) Dynasty, the separate walls were connected and consequently the wall stretched from east to west for about 5000 kilometres. This wall, considered to be one of the wonders of the world, served to keep nomadic tribes out. The Wall was further extended and strengthened by the succeeding dynasties. Now it is 6,700 kilometres in length.