Why did the Chinese rebuild the great wall of China in the 1400's

The first Emperor of China, Qin Shi Huang, is often considered the father of the Great Wall, but even before he united the nation in 221BC, individual states built walls to keep out invaders as early as the 7th Century. Qin connected, lengthened and fortified the walls to protect the northern border between 221-206BC. Jun 5, 2014

To keep out Northern savages. I think : P