why was the great wall of china built?

The Great Wall of China was built by the Qin Shihuangdi,
of the Qin Dynasty around 221 B.C.E.  The wall was primarily built to protect the
northern borders from the invading nomadic warriors from the river valley areas.
 Previous dynasty of Zhou had previous problems with the nomadic tribes coming into
contact with the settlements of the Chinese people.  There were actually previous
barriers in certain places.  Shihuangdi ordered they be connected to form one single
wall to protect the northern frontier.  The original wall stretched for 1,500 miles.
 The wall was built by forced labor including some government officials who resisted the
control of the Qin ruler.