The Second Punic War (218 to 201 BC) in which Hannibal had left a trail of devastation across Italy affected the town like many Latin colonies[4] and the rich bought up both public land and the small farms of the poor.

As part of land redistribution, a group of great villas were assembled in the area, run by slave labour like the latifundia estates typical of southern Italy.

Cosa appears to have been affected by an earthquake in 51, which occasioned the reconstruction of the republican Basilica as an Odeon under the supervision of Lucius Titinius Glaucus Lucretianus,[8] who also worked on the Capitoline temple.

[9] It is possible that the intermittent nature of the occupation of the town was due to the fact that, already in the early Empire, malaria was hyperendemic on the coast of Tuscany.

In the 20th century, Cosa was the site of excavations carried out under the auspices of the American Academy in Rome, initially under the direction of the archaeologist Frank Edward Brown.

From 2016, l’Università di Firenze has been excavating along the processional street P. Within the city walls the urban area was divided into an orthogonal plan, with space allotted for civic, sacred, and private architecture.

At the western corner of this was a postern, closed in the early Byzantine period, when the hill was refortified with a wall built with an emplecton.

The vast majority of religious monuments at Cosa were located at the Arx, "an area sacra, abode of those gods, quorum maxime in tutela civitas.

[20] Dating to the late 3rd century BC, Temple D was located opposite the north angle of the Capitolium's forecourt and was oriented SE.

Dyson holds that these evolving styles and similarities reflected the influence of the larger Hellenistic Mediterranean world that Rome was beginning to dominate.

[16] The forum was the public square of the city and was the site of many important structures, included a basilica and a curia-comitium complex, as well as buildings Brown termed atria publica, which have now been shown to be houses.

The forum of Cosa is fairly complex in archaeological terms and many of the Republican structures were later built over with constructions of the Imperial period.

In the '50s AD the site was hit by a substantial earthquake, and Atrium Building V, the 'House of Diana' was occupied by the man in charge of rebuilding, L. Titinius Glaucus.

A revival of activity occurred under Caracalla, when two substantial horrea were built, and the portico around the forum rebuilt, with a sanctuary to Liber Pater on the northeast side.

The building was identified when the area in front was excavated and found to be "a circle of dark earth enclosed by a sandy yellow fill".

However, to make sure he could build it, he replicated the concept of the Comitium and the Curia by placing a temple to Venus at the top of the theater with steps that doubled as seating.

Scholars speculate that these three halls are at the northern end a tabularium, with offices for aediles and other magistrates on the south side, and the Curia in the middle.

This occurrence of being tripartite is seen as a common aspect of Roman culture as well as in other areas of archaeology such as the later with the Curia Julia and around the 4th/3rd century BC with the south halls of the Forum at Pompeii.

The subsequent use of this site for a number of early medieval buildings has left little legible, but there remains enough to know that the podium, built of large ashlars like those of Temple D70 BCE, measured 6.25 x 11.25m.

In front, opening onto the forum, are two tabernae, with rear rooms and cesspits, probably intended for the sale of wine, in one case, and food, in the other.

[27] Elizabeth Fentress suggests that the differentiation in house size between the smaller plots in this block and those clustered around the forum is due to a colonist class distinction.

McCann points out that "the layer of mud deposited around and on top of the dock as well as the presence of many joining sherds suggests the possibility of destruction by a sudden disaster, such as a tsunami which swept into the inner lagoon.

[28] The city was likely founded in order to provide a strategically defendable port "close to the sources of timber and supplies from the Tuscan hinterland which would be necessary in the creation of the fleets Rome soon was to need in her first struggles for maritime supremacy with Carthage.

[29] The Cosa harbor was never a major port of transit, however in ancient times it provided the best anchorage between Gaeta in the south and La Spezia to the north.

[28] According to Anna Marguerite McCann, one of the later excavators, "in antiquity Cosa was a landlocked port, communicating with the sea by means of an artificial ship-channel, protected at the seaward end by a massive breakwater and provided with a set of elaborate channels cut into the limestone cliff, designed to keep the mouth free of sand.

"[28] Another lead excavator, Colonel John D. Lewis, invented two new technical devices: a water jet prober to help identify structures buried in the sand, and a construction of sheet steel formed by joining cylinders which could be used to obtain harbor stratification.

[28] Many fragments of transport amphorae have been found on the beach and offshore areas of the harbor, the earliest of which date to the late 3rd century.

[29] There is also a continuous foundation of stone (only visible underwater) for a breakwater which offered protection on the southern exposure, as well as a series of discontinuous extensions of the breakwater protecting the harbor from the south and southwest winds; "their spacing suggests that they were constructed with the primary purpose of breaking the crushing force of the seas without affecting the flow of currents in and out of the enclosed harbor area.

[29] Cosa appears in some documents dating from the 11th century, although a 9th-century occupation is suggested by frescoes at the abbey of S. Anastasio alle Tre Fontane in Rome, recording the capture of the site by Charlemagne and Pope Leo III.

Occupation of the site began with a few sunken-floored buildings, but by the 11th century it was concentrated on the Eastern Height, now surrounded by a double bank and ditch.