Short answers, pulled from the story.
Cosa was founded by Rome as a Latin colony in 273 BC on land confiscated from the defeated Etruscans. The Romans established it on the Ager Cosanus to consolidate control over the region and secure a strategically defensible port close to Tuscan timber supplies needed for building Rome's early naval fleets.
The major excavations at Cosa were conducted under the auspices of the American Academy in Rome, initially directed by archaeologist Frank Edward Brown in campaigns from 1948 to 1954 and 1965 to 1972. In the 1990s, Elizabeth Fentress led a follow-up campaign focused on the site's history between the imperial period and the Middle Ages. From 2005 to 2012, the Universities of Granada and Barcelona excavated a domus, and from 2013 Florida State University has excavated a bath building.
The Capitolium at Cosa is a triple-cella temple built in the 2nd century BC at the summit of the Arx, the city's sacred citadel. It is believed to be the only Capitolium known to have been constructed in a Latin colony, modeled after the 6th-century BC Temple of Jupiter, Juno, and Minerva in Rome. Situated high enough to be visible for miles at sea, it was likely built as an affirmation of Roman loyalty following the Second Punic War.
The port of Cosa, located below the ancient city on the Tyrrhenian coast of Tuscany, is identified as the earliest Roman harbor known thus far. It was likely founded alongside the colony in 273 BC and remained in use into the 3rd century AD. The port also preserves the earliest known commercial fishery and the earliest evidence for the use of tufo and pozzolana concrete in water.
The Sestius family were major exporters of wine whose stamped amphorae have been found in large numbers at the port of Cosa, with the earliest examples dating to 175-150 BC and continuing into the 1st century BC. The abundance of Sestius amphora fragments at the harbor suggests that Cosa was the primary center for manufacturing and distributing these famous jars, with the family's trade network extending as far as Gaul.
Scholars attribute the intermittent occupation of Cosa partly to hyperendemic malaria on the Tuscan coast, which may have made sustained habitation difficult from the early Empire onward. The city also suffered from the disruptions of the Roman Republican civil wars in the 60s BC, a major earthquake in 51 AD, and periodic economic decline. By AD 417, the traveler Rutilius Claudius Namatianus recorded that the site was deserted and in ruins, suggesting a plague of mice had driven away the last inhabitants.