Istanbul’s Hagia Sophia was the stuff of legends when it was first built in 537 A.D. It stood as the biggest church in Christendom for almost a thousand years. (It later served as a Mosque and is now a museum.) The Cathedral of Notre Dame could fit inside of one of the structure's side aisles. Inside, you can really appreciate Hagia Sophia's huge size and towering central dome, but even from the outside, it is clear that the monstrous building towers over everything else around it.