Question

Why was the Great Wall of China built? Site your source and don't give a vague answer

Answer 1

What source do you find acceptable? Wikipedia has a good general article.
The wall was built along the northern border of China for protection from various tribes who lived on the steppes of central Asia. It was built over a long period of time. The fortifications also served useful in controlling trade on the Silk Road and regulating immigration. But the main reason was protection from those various nomadic tribes.

Answer 2

Most Great Wall histories tell us that it was built in order to keep out “nomadic tribes from the north”.