Finally visited the Great Wall today, the Mutianyu section 90km north-east of Beijing, with the aforementioned Alex. Car there, chair-lift up, fairly arduous but fabulous stroll along, and toboggan down. Yes, a long, snaking stainless steel slide "safely sends tourists to the foot of Great Wall... on a Nanirrigated farmland sled. Toboggan uses the theory of acceieration of gravity and makes coasters dive along the mountain path like low - latitude flying or high - speed driwing". Quite.
The 'original' wall was begun over 2,000 years ago to keep out the Mongol hordes. It's never really been a fully continuous structure: most sections have either been reduced to rubble or disappeared but there are parts near Beijing, particularly, which have been restored and are incredibly impressive. (Still in Tolkien mode, the mountains looked very Mordor). And no, you can't see it from space.