Numerous remains testify to the city's past greatness. On Ortygia are the cathedral, built (7th cent. A.D.) on the remains of a Greek temple, with 12 Doric columns; the remarkable archaeological museum; the fountain of Arethusa; ruins of a temple of Apollo; and a castle built (13th cent. A.D.) by emperor Frederick II. Among the remains on the mainland are a large, well-preserved Greek theater (5th cent. B.C.), still used for performances of classical works; a Roman amphitheater (2d cent. A.D.); the large Greek fortress of Euralus; and the extensive Catacombs of St. John (5th–6th cent. A.D.).