Funny thing is, although you can't see the Great Wall of China from space, there are a few manmade structures you can. If you include a city as a manmade structure, then that is visible. The pyramids are just on the cusp.
If you think about it, the Great Wall is only great in one direction, its length. It isn't so tall that it would stick up above the landscape or cast a big shadow. It isn't so wide that you would see it looking down. If something is only big along one dimension you won't see it!
Runways are pretty easy to see from space, if you remember they are a good hundred metres wide and a couple of miles long and are usually a totally different colour to everything else around it.