Built in the late 3C and based on the Pantheon in Rome (1C), the huge circular building that is the rotunda gives an impression of serene power. It is not known whether it was originally a pagan temple dedicated to Zeus or a mausoleum erected for the Emperor Galerius, but it was converted into a baptistery in the 4C, became a cathedral in 1524 and a mosque in 1591. It was deconsecrated in 1912. The mosaics on a gold background in the nave (6C) are masterpieces of early Christian art.