The octagonal Florence Baptistery, located in front of the famous Duomo, was built between 1059 and 1128 and is one of the oldest buildings in the city. It is the spiritual heart of Florence and at one time every child born in the city was baptized inside its great stone font. The Baptistery is renowned for Ghiberti's 15th century bronze doors with relief sculptures - considered so beautiful that Michelangelo christened them the "Gates of Paradise" - and his creation is often cited as the dawn of the Renaissance. The interior is decorated with a shimmering gold mosaic and features the tomb of the pirate pope, Baldassare Cossa, made by Donatello and paid for by the Medici family. The designs for the mosaics were provided by some of the finest Florentine artists of the pre-Renaissance era. Work began on the arch over the altar in 1225 and continued to cover the entire roof vault during the next hundred years or so.