They used to say the urban myth when I was a kid that the Great Wall was the only man-made object that was visible from space. This isn't true at all -- it's far too narrow. Light pollution, however, means that every metropolis on the planet can be seen at night from the moon, but it was a good way to give the impression of how massive it was. Apparently the myth began in the late 1800s before space flight even existed. But what is amazing is how the wall is so long and so old and yet it still exists (despite Mao's Cultural Revolution habit of encouraging villagers to take bricks and incorporate them into their homes because it was a symbol of despotism).
Whether in summer when the humid hot haze makes climbing unbearable, or after a recent snowfall when the icy-trodden steps are treacherous, the Great Wall is an amazing place. Counting watch towers becomes a rite of passage for many! There are so many different sites to visit, and most of the more famous ones are near to Beijing, and each is unique. Whether it's 30 towers over hours end to end in Simatai, or the most-visited section, Badaling, with its cable car up. Mutianyu has a chairlift up and fun metal track sled ride down. The majestic Jinshanling which is connected to Simatai has spectacular views over a town where the wall comes down to the valley floor.
Built over centuries, over a million people are supposed to have died in its construction. Many other myths about about the historical structure, some of which can be read here. The coolest legend I heard is that it's path was laid out by a helpful dragon.
Simatai September 2003
Badaling March 2007 and Mutianyu March 2009
Jinshanling March 2010Sources: http://facts.randomhistory.com/2009/04/18_great-wall.html; http://en.wikipedia.org/wiki/Mutianyu; http://en.wikipedia.org/wiki/Jinshanling; http://en.wikipedia.org/wiki/Simatai; http://en.wikipedia.org/wiki/Badaling