The Duomo of Siracusa represents layers of history. The foundation–both base and walls of the structure–is an ancient, Doric-style Temple of Athena, built in the 5th century BCE (Once inside, you can easily see the columns of the Greek temple!). During the Byzantine age in the 7th century CE, the temple was converted into a Christian basilica, and its front became its back.
After being sacked by the Arab conquerors in 878 and its Norman-era steeple and stone roof collapsing with the historically powerful earthquake of 1693, the current baroque-style cathedral, dedicated to the Navtività di Maria Santissima, was constructed at the beginning of the 18th century. The Duomo holds court on the Piazza del Duomo of Ortigia, a site I think to be one of Italy’s most breathtaking public spaces.