Can someone preferably Chinese would tell me what was the main purpose to build 'Great wall of china'?

Can someone preferably Chinese person (If not Chinese then anyone who has knowledge of) would tell me what was the main purpose to built 'Great wall of china'?Who built it and why?

i am chinese, i learn that in histry lesson when i am in high-middle school , the main pourpose to buit the great wall is to defend the mongolia army ,the great wall has buit 2 time.

the first time is buit by china emperor whos name is 绉﹀鐨?--
QIN SHI huang, in the MING dinasty----锛堟槑鏈濓級the emperor name is ZHU WON ZHANG ----鏈卞厓鐠嬶紝

I am a chinese, but i think u cud check ur facts at wikipedia, I dunno who built it, but it was built to stop the mongols from conquering china

It was a boundary marker meant to keep the bad guys out. Also meant to contradict modern day Chinese warhawks that claim ownership of the sovereign nation of Tibet. The great wall of China does not include many of the countries that modern dat China now claims to own. If you click on the wikpedia map below you can see that the wall is about a thousand miles away from Tibet so you see the evil people lie when the claim to own some one else's country when their own ancient border really is etched in stone.

The Great Wall served as a military defense border to keep invaders, and their horses, out. It allowed a relatively small number of Chinese to protect a very long border. It enabled transportation (by foot and on horseback) and communication (by smoke signals in the daytime and fires at night) along the border. And it served to intimidate and discourage would-be invaders.

It was built to defend the country from being attacked by invaders. Soliders stationed on the great wall to guard the place.