04-25-2013, 05:05 AM
50 km is described as the edge of space, at least as far as Earth is concerned; but there is still a significant atmosphere at that level, and losses would be quite rapid if the walls were only that high. On the other hand 200 km walls would block out quite a lot of sky as seen from the ground- one might as well lie in a sealed torus. Perhaps the top of the wall could be made transparent somehow.