It's located in what is the modern day Ansedonia, a short distance from Orbetello and overlooking the Tyrrhenian Sea.
The Romans had a simple reason for building Cosa – it allowed them to control both the land and sea traffic that passed through Tuscany on its way to Rome.
Of course the land wasn't empty when the Romans took it. It has been ruled by the Etruscans of Vulci before they'd be resoundingly defeated by the Romans.
The Roman city was built between two hills. Politics was given its own hill with the Forum, while the other held the Acropolis, where citizens went to worship the gods.
The massive walls that surround the town show that, at least at the time of the foundation, the Romans weren't convinced they'd completely subdued the local Etruscans, who were still hostile.
But in the end it wasn't the Etruscans that destroyed Cosa. The city was destroyed by pirates in 70BC. A few people returned 50 years later, but the city never regained its former glory.
In the Middle Ages, a community moved into Cosa, pottering around the area and building a garrison and a church with adjoining cemetery.
As you approach Cosa, you can see the mix of buildings from later centuries that slowly encroached on the Roman ruins before it was made a protected archaeological site.
Today Cosa is incredible. Its ruins are more modest than what you'll find in Rome, but still beautiful to behold and located in a gorgeous stretch of countryside, looking out to sea.
Unlike in the capital, you are free to walk among these ruins. Explore the ancient villa in the Forum or the public baths and really soak up the history.
Admittedly the site is a bit of a mess and I use the term museum very lightly. To be honest, you're on your own when you visit Cosa.
There is a small museum with a collection of treasures gathered from Cosa, but there's a definite sense the best bits have been carted off to display elsewhere. Entry is €2.
The archaeological site and the ruins of the city are free to visit. If you're unsure about how to get to Cosa, stop at the tourist information centre in Piazza Repubblica in Orbetello for directions.
Photo: Institute for the Study of Ancient Worlds via Flickr.
Institute for the Study of the Ancient World
Address: Via delle Ginestre, Ansedonia
Opening hours: Mon-Sat: 8:30am-7:30pm
Price: entry into the museum is €2, the archaeological sites are free
Tempio di Giove
The Temple of Jupiter was built in 273BC, making it the oldest building in this ancient Roman colony.