A surreptitious photo from the balcony. The building is older than the light, airy Gothic cathedrals, so the columns are thick and heavy to accommodate the weight of the domes. The interior is relatively dark, although not as dark as the books introducing the advances of Gothic architecture would suggest (and likewise, the Gothic cathedrals in Florence did not seem terribly bright, and I ended up taking grainy pictures using ISO 3600 in many of them).

Despite the heaviness, the architecture is really well done. The building is essentially domes on top of domes on top of arches, so you can see the weight flow elegantly to the thick columns on the floor.

The floor is filled with geometric mosaics. It is also very badly warped, from the weight of the basilica settling unevenly.