The May 21 New Yorker carried a "Letter from China" by Peter Hessler titled "Walking the Wall; Can one man's obsession unravel the mystery of an ancient structure?". It's about an American (David Spindler) researching a book on the Great Wall of China. Believe it or not, there are no exhaustive studies on the topic--from any era. Scholars have tended to neglect it. "There isn't a scholar at any university in the world who specializes in the Great Wall of China."
I learned that there isn't one continuous wall (which, by the way, cannot be seen from the moon) but rather many different lines of fortifications. These were built to repel Mongol and Turkic tribes. Here's a bit of history (I have to type it):
In 221 B.C., Qin Shihuang became the first ruler in Chinese history to declare himself emperor. After consolidating power, he commanded the construction across the north of roughly three thousand miles of changcheng. The term translates as either "long wall" or "long walls"--Chinese doesn't differentiate between singular and plural--and the barriers consisted of hard-packed earth. Over the centuries, many other dynasties faced the same basic problem as the Qin: the wide-open frontier of the northern plains made them vulnerable to the nomadic Mongol and Turkic tribes that inhabited these lands. The nomadic threat was more intense in some periods than in others, and Chinese dynasties responded with different strategies. The Tang, who ruled from 618 to 907 A.D., built virtually no walls, because the imperial family was part Turkic and skilled in Central Asian warfare and diplomacy. Even when dynasties constructed walls, they didn't necessarily call them changcheng; more than ten terms were used to describe the fortifications.
The Ming usually called theirs bianqiang--"border wall(s)"--and they became the greatest wall builders in Chinese history. They came to power in 1368, after the collapse of the Yuan, a short-lived Mongol dynasty that had been founded by Kublai Khan. The Ming constructed large fortifications of quarried stone and brick in the Beijing region--these are the iconic structures (some of them rebuilt and restored) that seem to continue endlessly in tourist photographs. They were the only dynasty to build extensively with such durable materials, and many sections of the Ming wall ran for miles. But the bianqiang was a network rather than a single structure,and some regions had as many as four distinct lines of fortifications. . . .
Sounds like the Tang had the right idea. Love your enemy, know how to fight like them, be diplomatic, and you don't have to build wall(s).*
*Sounds like a lesson this country could learn.