The Great Wall of China

We visited the great wall today, and were blessed with clear blue sky and a temperature that was bearable, with a few layers.   Took a cable car about 2/3 of the way up and hiked the last 1/3... which got us warmed up.  I was wondering how the ming emporers convinced their people to spend so much effort to build the this latest version of the wall.  I was wondering how much of China's GDP at that time was spent on the wall compared to what the US spends on defense/offense.  It all seems like a bit of a waste and does not really work in the long run. 

 Took a nice group pic.  Emily (and others) then decided to do a "I wonder if i can hurt myself" jumping picture.  See attached.