The Cathedral of Siracusa is one of the most extraordinary buildings in Italy — not for the Baroque facade that faces the piazza (though it is fine), but for the interior, where the columns of a 5th-century BC Greek temple to Athena are incorporated wholesale into the church walls. You can stand inside a room that was sacred to Athena, then to the early Christians, then to the Arabs, then to the Normans, and finally to the Baroque Catholicism that gave it its current form.
The building occupies the highest point of Ortigia, where the Greeks would have placed their principal temple deliberately. The piazza outside — the Piazza del Duomo — is the finest public space in Siracusa and arguably the most beautiful baroque square in Sicily. Sit at one of the cafe tables in the evening and watch the light change on the buildings opposite.