In 509 BC, after the overthrow of the Roman monarchy, the family of Tarquinius Superbus went into exile in Caere in Etruria. Tarquin sought to regain the throne, at first by the Tarquinian conspiracy and, when that failed, by force of arms. He convinced the cities of Tarquinii and Veii to support him, and led their armies against Rome in the Battle of Silva Arsia. Although the Roman army was victorious, it is recorded by Livy that the forces of Tarquinii fought well on the right wing, initially pushing back the Roman left wing. After the battle the forces of Tarquinii returned home.
In 358 BC, the citizens of Tarquinii captured and put to death 307 Roman soldiers; the resulting war ended in 351 BC with a forty years' truce, renewed for a similar period in 308 BC. When Tarquinii came under Roman domination is uncertain, as is also the date at which it became a municipality; in 181 BC its port, Graviscae (mod. Porto Clementino), in an unhealthy position on the low coast, became a Roman colony. It exported wine and carried on coral fisheries. Nor do we hear much of it in Roman times; it lay on the hills above the coast road. The flax and forests of its extensive territory are mentioned by classical authors, and we find Tarquinii offering to furnish Scipio with sailcloth in 195 BC. A bishop of Tarquinii is mentioned in 456.
The original residential quarters of the Etruscan city of Tarquinia, known as the "Civita", were on the long plateau to the north of the current town. The ancient burial grounds, dating from the Iron Age (9th century BC, or Villanovan period) to Roman times, were on the adjacent promontories. The ancient city had shrunk to a small fortified settlement on the "Castellina" location during the early Middle Ages, while the more strategically placed Corneto (possibly the "Corito" mentioned in Roman sources) grew progressively to become the major city of the lower Maremma sea coast, especially after the destruction of the port of Centumcellae (modern Civitavecchia). The last historic references to Tarquinia are from around 1250, while the name of Corneto was changed to Tarquinia in 1922. Reversion to historical place names (not always accurately), was a frequent phenomenon under the Fascist Government of Italy as part of the nationalist campaign to evoke past glories.
The Etruscan necropolises, with some 6,000 tombs, 200 of which include wall paintings. The main site is the Necropolis of Monterozzi, with a large number of tumulus tombs with chambers carved in the rock. The scenes painted include banquets, with dances and music, sporting events, occasional erotic and mythical scenes. There are also carved sarcophagi, some dating to the Hellenistic period. Famous tombs include the Tomb of the Bulls, Tomb of the Augurs and the Tomb of the Leopards.
Remains of the so-called Ara della Regina temple (original dedication unknown), measuring c. 44 × 25 m and dating to c. 4th-3rd century BC. It was built in tufa with wooden structures and decorations, notably a frieze of winged horses in terracotta. Also traces of the Etruscan walls (c. 6-4th century BC) exist: they had a length of some 8 kilometres (5 miles).
The Tarquinia National Museum, with a large collection of archaeological findings. It is housed in the Renaissance Palazzo Vitelleschi, begun in 1436 and completed around 1480–1490
Church of Santa Maria di Castello (1121–1208), with Lombard and Cosmatesque influences. The façade has a small bell-tower and three entrances. The interior has a nave and two aisles, divided by massive pilasters with palaeo-Christian capitals and friezes. Noteworthy are also the rose-window in the nave and the several marble works by Roman masters.
The Cathedral, once in Romanesque-Gothic style but rebuilt after the 1643 fire, has maintained from the original edifice the 16th-century frescoes in the presbytery, by Antonio del Massaro
Church of San Giacomo and Santissima Annunziata, showing different Arab and Byzantine influences
The small church of San Martino (12th century)
The church of St. John the Baptist (12th century), with an elegant rose-window in the simple façade.
The Communal Palace, in Romanesque style, begun in the 13th century and restored in the 16th