The Baptistry of San Giovanni is thought to be the oldest monument in Florence. The first known mention of it was in a document dated 897, but the exact date of its construction is not known.
In the Middle Ages, it was believed that it had been a temple to Mars in the Augustan age, which was then transformed into a Christian church dedicated to John the Baptist around about 310. This hypothesis has not been confirmed historically, but it is certain that the "Bel San Giovanni", as Dante called it in the Inferno, is very dear to the Florentines' hearts; San Giovanni is the patron saint of the city (celebrated on June 24), and the effigy of the saint was also reproduced on the ancient Florentine coin, the florin.
On November 6, 1059, the Baptistry was reconsacrated by the Pope and Bishop of Florence, Nicola II, probably while the old building was being enlarged, with the addition of the third tier and the construction of the pyamidal roof.
At first, there were steps up to the edifice, but these disappeared as the level of the street gradually rose.
In the interior of the Baptistry, besides the spectacular marble floor (partially tessellated with signs of the zodiac), the XIV century baptismal font of the Pisan school, and the precious mosaics inside the dome, there are also other extremely significant works of art.
The building has a characteristic octagonal layout, and is known throughout the world for the magnificence of its three sets of carved bronze doors; positioned according to the cardinal points, these carvings recount the history of humanity and of the Redemption.
The central door has scenes from the Old Testament, while the south door recounts the history of St. John the Baptist. Finally, the north door relates the story of Christ. These doors, which replaced older ones in wood, were produced by supreme artists: the south doors (the oldest) are the work of Andrea Pisano, the north doors (also called the Doors of the Cross), were created by Lorenzo Ghiberti, as were the east doors, usually referred to as the Doors of Paradise.
Although it is included in the area of Piazza Duomo, the Baptistry has another piazza in front of its entrance, Piazza San Giovanni, which was enlarged many centuries ago with the setting back of the Palazzo dell' Arcivescovado.