Cosa was a Latin colonia founded under Roman influence in southwestern Tuscany in 273 BC, on land confiscated from the Etruscans. The Etruscan site (called Cusi or Cosia) may have been where modern Orbetello stands; a fortification wall in polygonal masonry at Orbetello's lagoon may be in phase with the walls of Cosa. The position of Cosa is distinct, rising some 113 metres above sea level and is sited 140 km northwest of Rome on the Tyrrhenian Sea coast, on a hill near the small town of Ansedonia. The town experienced a hard life and was never truly a prosperous Roman city, although it has assumed a position of prominence in Roman archaeology owing to the circumstances of its excavation.

“He who has not seen the so-called Cyclopean cities of Latium…those marvels of early art, which overpower the mind with their grandeur, bewilder it with amazement, or excite it to active speculations as to their antiquity, the race which erected them, and the state of society which demanded fortifications so stupendous on sites so inaccessible as they in general occupy; — he who has not beheld those sublime trophies of early Italian civilization — the bastion and round tower of Norba — the gates of Segni and Arpino — the citadel of Alatri — the many terraces of Cora — the covered way of Praeneste, and the colossal works of the same masonry in the mountains of Latium, Sabina, and Samnium, will be astonished at the first view of the walls of Cosa.

Nay, he who is no stranger to this style of masonry, will be surprised to see it on this spot, so remote from the district which seems its peculiar locality. He will behold in these walls immense blocks of stone, irregular polygons in form, not bound together with cement, yet fitted with so admirable nicety, that the joints are mere lines, into which he might often in vain attempt to insert a penknife: the surface smooth as a billiard-table; and the whole resembling, at a little distance, a freshly plastered wall, scratched over with strange diagrams.”

―George Dennis, The Cities and Cemeteries of Etruria, London, 1848