"Angelic and non-human design," was how Michelangelo described the Pantheon 14 centuries after its construction. The highlights are the gigantic dome, the upper eye, the sheer size of the place, and the harmony of the whole building. We visited with a Roman guide which is exactly how you should (or shouldn't) visit the city, especially since they talk too much and have lots and lots of history of each building.
And so we learned things like how the interior dome was filled with sand during construction. Or not, because it really is impossible to be sure of anything when one doesn't know Italian and the guide has a very thick Roman accent.