This is the famous Duomo, also known as the Cathedral of Florence. The amount of artwork and detail on the exterior is simply amazing, and must been seen first hand to really appreciate its splendor. You can also walk up a series of narrow winding steps to the top to get a great view of the city. As it turns out, Siena and Florence are (and have been for centuries) rivals. The Duomo in Siena is even more spectacular.