Desolately the wind rises.
We march thousands of miles over vast distances.
Why do we cross the deserts?
To build the Great Wall.
This poem, written by Emperor Yangdi at the beginning of the seventh century A.D., is a lyrical reminder of the centuries spent by the Chinese building a wall to repel foreign invaders. Just one among many defensive strategies that China employed, the Great Wall stretches more than 5,000 miles long. Rather than one continuous wall, it consists of many smaller pieces, all built during different eras in history. (See also: Building walls may have allowed civilization to flourish.)
The earliest fortifications date back as far as the seventh century B.C., but the best-preserved sections were built during the Ming dynasty (1368-1644). The Great Wall is a marvel of engineering and triumph of human ingenuity, but the verdict is out on how well it worked at its primary function: keeping people out.