The Baptistery of San Giovanni was founded on 1152, and was used to administer the Sacrament of…read moreBaptism. It was meant to be mirror the majesty of the adjacent Cathedral and is indeed the largest one in Italy (couldn't find any mention what is the biggest in the world).
A ticket is required to enter. (7€ individually, or 10€ if part of a pack to see the other 4 museums on the campus) It is one giant dome, with a visitor accessible second level overlooking the first floor. The giant baptismal fountain isn't really admirable from ground level, but from the upper level, it is something. It is 69 steps going down (forgot to count going up...probably similar)..no elevator.
This is the one building where you can pay extra (2€) to listen to an audio guide.
While we were up there, a guide began to sing a chant to publicize and demonstrate the amazing acoustics of the dome. I've read that the building can be considered one giant musical instrument.