The ancient city of Cosa is a Roman settlement located in Ansedonia, in the municipality of Orbetello, in an elevated position that enjoys a splendid view of the entire Argentario and the Island of Giglio. The National Museum of Archeology conserves artefacts coming from this Latin colony founded in 273 BCE. The city was surrounded by a large wall 1.5 kilometres in length and with three doors and many towers: a second wall bordered the acropolis, with an opening toward the port.
At the beginning, the museum was just one room dedicated to the most important artefacts found mostly at the citadel, the forum and in private houses, like the clay decorations from the temples at the citadel, in addition to ceramics, glass and objects made from metal and ivory.
Today, there are two new exhibition rooms: the first is dedicated to the port area, trading and the artefacts found in the necropolis located around the city, while the second room displays the history of Cosa until the 15th century.