The ancient Roman city of Cosa stands on a rocky promontory formed by two hills, separated by a vast saddle. Founded by the Romans in 273 BC. after the victory over the allied forces of the Etruscan cities of Volsinii and Vulci in 280 BC, it included a large part of the Vulci territory, including the coast. The Latin colony of Cosa thus controlled a large geographical area. The name derived from the older one of Cusi or Cusia, referring to a small Etruscan center located in present-day Orbetello.
The strategic position and nature of a fortress, evident from the imposing walls, were necessary to face the threat of Carthaginian naval power and to maintain control over the newly conquered Etruscan territories. In the urban heart, a dense grid of streets intersect at right angles, defining rectangular blocks for settlers' homes and larger areas for public buildings. The city's port, Portus Cosanus, was built at the foot of the promontory, with a coastal lagoon behind it, today represented by Lake Burano. Between the 12th and 14th centuries, the region came under the alternating rule of the Aldobrandeschi, the Republic of Orvieto and finally the Republic of Siena, which destroyed it in 1329.