The Pantheon was one of our favorites!  The fact that it still stands after being built in the first century BC - an that it was built at all considering it's poured concrete roof and everything - is amazing to us.  Being given over to the church helped it to survive as well.  You see the oculus in the ceiling here - the only source of light for the whole building.  The ceiling was coffered to cut down on it's weight since it is concrete.  The altar still stands in place as well.  A beautiful building - an amazing feat of construction!  The fountain sits in front of the Pantheon in Piazza della Rotunda.