This baptistery is where people baptized their children in medieval Pisa (PEE-za), in Italy. The people of Pisa built their round baptistery in the Romanesque style around 1150 AD. So the lower levels have round arches. But by the time they finished building the upper part, architects were changing over to the Gothic style. So they made the upper windows with the new fashionable pointed arches.
Compare this baptistery to the baptistery in Florence, very close by. The people of Florence built their baptistery just a little earlier.