Some facts: The Great Wall (长城, cháng chéng) stretches nearly 6,000 miles. It was built and rebuilt over about 2,000 years, from as early as the 7th century BC in the Warring States Period through the Ming Dynasty (1600s AD). It runs east-to-west along the historical northern border of China, serving as a barrier against the northern barbarians (ultimately unsuccessfully), as well as a way to control trade along the Silk Road and immigration across the border.
Today, the Great Wall is in mixed condition. Some areas have been rebuilt yet again, not for any protective reason but to commercialize for tourists. Roughly thirty percent is said to be in ruins; nearly fifty percent -- and growing -- has disappeared entirely. Some stones are lost by attrition, as Chairman Mao encouraged farmers to use stones from the Wall as building material. Others are lost to the elements, due to erosion by sand and collapse from rain.
It is impressive to climb the steps, stand upon the Wall, and look out to see it snaking through the mountains, watch towers appearing regularly along the ridge. It is even more impressive to consider the conditions under which it was built -- purely by manpower, without any of the machinery we'd consider essential today. The Great Wall continues to hold the title of the world's largest manmade object.
The Mutianyu (慕田峪) section of the Wall has gained some modern concessions for the hordes of tourists. Rather than climb all the steps to the top, people can opt to ride the chairlift. On the way down, there's a toboggan slide (note: speed of descent depends heavily on the people in front of you. One Chinese grandmother brought us all to a crawl).
I hope that someday we make it to Shanhaiguan, the eastern terminus of the Great Wall, where the Wall literally runs into the sea.