It does look like that does it. Although it' very well kept for an "ancient" civilization.

But to me it looks like a modern/post-modern squad of troops stumbled across an inhabited civilization less advanced than the one these troops came from. Judging by the floating islands in the distance, it's definitely not on earth. Who knows who could be living there, aliens/humans that lived in another planet (Think of battlestar in where humans didn't originate on earth but on another planet.) like an ancient and forgotten colony world who had major technological regression and was rediscovered.