This is where Catholic priests baptized Christian babies in medieval Florence. Probably people had built an earlier, smaller octagonal baptistery here around 700 AD. Catholics may have built the earlier baptistery to encourage Lombards to convert to Catholicism from Arianism.
But that baptistery got old, and too small. The people of Florence built this new octagonal baptistery in the Romanesque style around 1100 AD. The builders covered the outside with patterns of red, white, and green marble. The architect probably learned this way of patterning buildings from earlier buildings in the Islamic Empire, across the Mediterranean from Florence. For example, the Great Mosque at Cordoba and the Dome of the Rock mosque in Jerusalem, from the 600s-700s AD, both have stripes like these.
The octagonal/round shape was also common in earlier medieval buildings. The first one was the mausoleum of Galla Placidia in Ravenna (ca. 450 AD). Then Hagia Sophia and Hagia Irene in Constantinople from the 550s both had octagonal or round parts. A little later, the Dome of the Rock in Jerusalem (600s AD), and Charlemagne’s chapel at Aachen (ca. 800 AD) were octagonal or round again.
Most of these buildings were covered with mosaics on the inside walls. And so is the baptistery of Florence. The mosaics here were done in the 1200s AD. Visiting artists from Venice made some of the mosaics, and artists from Florence like the famous Cimabue (chee-MA-boo-ay) did others.
The baptistery has fancy bronze doors on three sides. The artist Pisano molded one set of doors, using lost wax casting, in 1330 AD. Ghiberti molded the other two sets of doors in the 1400s. Ghiberti (gi-BEAR-tee) won a competition against Michelangelo to see who would get to carve these doors when he was only 23 years old. I think looking at this panel of Cain and Abel you can see why he won!
You might want to compare this baptistery to the baptistery in Pisa, very close by, which the people of Pisa built just after this one. The Florence baptistery is just across from the later Duomo, or Cathedral of Florence.