The odor of sanctity. Pretty small church all in all. Though it is open all the time, and there are always people praying in the little chapels along the sides.
It doesn't look all that impressive from the outside. But then, that's the Roman way to do things...either its a honkin great monument, or they keep it all inside. You can tall the tourists, women with bare arms, men with puzzled looks and maps.
What can I say? Sculpture and architecture by Bernini, art work by Carvaggio. My goodness! Names which rank up there with the best! The list of men involved in the building of this church read like a who's who of the Renaissance!