To stop Mongols from the north, they built barriers seen by some as early precursors to the Great Wall of China, built later in 220 B.C.
Protected by the Himalayan Mountains, Pacific Ocean and Gobi Desert, and situated between the Yellow and Yangtze rivers, the earliest Chinese civilizations flourished in isolation from invaders and other foreigners for centuries. To stop Mongols from the north, they built barriers seen by some as early precursors to the Great Wall of China, built later in 220 B.C.