Averaging 23 feet high and 21 feet wide, the walls 5500 miles were punctuated by watchtowers.
长城平均高7米，宽6米，5500英里的城墙中间隔着瞭望台。
When raiders were sighted, fire and smoke signals traveled between towers until reinforcements arrived.
一旦看到有侵入者，瞭望台之间便会用升起的狼烟与风火传递信号，直到援兵抵达。
Small openings along the wall let archers fire on invaders, while larger ones were used to drop stones and more.
城墙上的小孔让弓箭手能够射击侵入者，而大孔则用于投掷石头或者其它东西。
But even this new and improved wall was not enough.
但即便是这种全新改进的城墙仍然不够好。
In 1644, northern Manchu clans overthrew the Ming to establish the Qing dynasty, incorporating Mongolia as well,
1644年，北方满族推翻明朝建立清朝，同时合并了蒙古。
Thus, for the second time, China was ruled by the very people the wall had tried to keep out.
因此，这是中国第二次被城墙抵御的外族统治。
With the empire's borders now extending beyond the Great Wall, the fortifications lost their purpose.
随着帝国边界扩展到长城以外，这些工事失去了原本的目的。
And without regular reinforcement, the wall fell into disrepair, rammed earth eroded, while brick and stone were plundered for building materials. But its job wasn't finished.
没有了定期加固，城墙失修破损，夯土被侵蚀，砖和石头则被盗窃用作建筑材料，但是长城的使命仍未结束。
During World War II, China used sections for defense against Japanese invasion, and some parts are still rumored to be used for military training.
第二次世界大战期间，中国用部分城墙来抵御日本侵略，一部分城墙仍然被盛传用于军事训练。
But the Wall's main purpose today is cultural.
然而今日，文化才是长城的主要目的。
As one of the largest man-made structures on Earth, it was granted UNESCO World Heritage Status in 1987.
作为地球上大型人工建筑之一，长城于1987年被联合国教科文组织列为世界文化遗产。
Originally built to keep people out of China, the Great Wall now welcomes millions of visitors each year.
长城起初建造用于防御外人进入中国，如今每年都会迎接数百万游客。
In fact, the influx of tourists has caused the wall to deteriorate, leading the Chinese government to launch preservation initiatives.
实际上，大量涌入的游客已导致长城日益损坏，致使中国政府启动保护措施。
It's also often acclaimed as the only man-made structure visible from space.
它也经常被宣称为唯一一处可自太空看见的人造建筑。
Unfortunately, that's not at all true.
不幸的是，这种说法不完全正确。
In low Earth orbit, all sorts of structures, like bridges, highways and airports are visible, and the Great Wall is only barely discernible.
在地球低轨道上，各种建筑物如桥梁、高速公路以及机场均可看见，而长城却若隐若现。
From the moon, it doesn't stand a chance.
从月球上，它则一点也看不到。
But regardless, it's the Earth we should be studying it from because new sections are still discovered every few years, branching off from the main body and expanding this remarkable monument to human achievement.
但无论如何，地球才是我们应该好好研究长城的地方，因为每隔几年都会有新的部分被发现，它们从主体部分扩展开去，不断延伸着这见证人类成就的伟大奇迹。
