Circeii, an ancient city of Latium, situated at the N. foot of Mons Circeius (now the mountain promontory Circello), near the sea, 55 m. S. E. of Rome. It was colonized by the Romans in the reign of Tarquin the Proud, who deemed its situation advantageous for commerce, and also wished to make it a bulwark against the Yolscians. It is mentioned among the conquests of Coriolanus, and soon after fell into the hands of the Yolscians, but was reconquered by the Romans. Circeii subsequently revolted from Rome, joined the Volscians, succeeded in establishing its independence, appeared as one of the cities of the Latin league at the outbreak of the great Latin war in 340 B. C, and furnished one of the two pnetors who commanded the whole nation. It appears again among the 30 Latin colonies in the second Punic war, but from this time seems to have declined, and to have become gradually insignificant. It was subsequently, under the later republic and the empire, a favorite place of retirement for wealthy Romans, having a peculiar attraction in the abundance and excellence of its oysters. It was sometimes selected as a place of exile, and the triumvir Lepidus was banished hither. Its ruins are still to be seen on the Monte della Cittadella, about 2 m. from the sea.
The remains of its walls, constructed of polygonal blocks, show a massive style of architecture.