The Istrian peninsula was conquered by the Romans in 177 B.C., starting a period of Romanization. The town was elevated to colonial rank between 46-45 B.C. . During that time the town grew and had at its zenith a population of about 30,000 It became a significant Roman port with a large surrounding area under its jurisdiction. During the civil war of 42 B.C. of the triumvirate of Octavian, Mark Antony and Lepidus against Caesar's assassins Brutus and Cassius, the town took the side of Cassius, since the town had been founded by Cassius Longinus, brother of Cassius. After Octavian's victory, the town was demolished. It was soon rebuilt at the request of Octavian's daughter Iulia and was then called Colonia Pietas Iulia Pola Pollentia Herculanea'. Great classical constructions were built of which a few remain. The Romans also supplied the city with a water supply and sewage systems. They fortified the city with a wall with ten gates. A few of these gates still remain: the triumphal Arch of the Sergii, the Gate of Hercules (in which the names of the founders of the city are engraved) and the Twin Gates. During the reign of emperor Septimius Severus the name of the town was changed into "Res Publica Polensis"
In 425 A.D. the town became the center of a bishopric, attested by the remains of foundations of a few religious buildings.
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