Cosa was designed for a large number of people and thus had pretty expansive city walls. They were built in the 3rd century along with the rest of city.
The original walls didn't just protect Cosa from land invasions, but also from sea incursions. The perimeter extended for almost a mile and was reinforced by 18 towers for sighting, and defense and offense action.
A thousand years after site was abandoned, the Aldobrandeschi came along and according to local reports, found an imposing city wall that didn't require any changes on their part.
Of course, they still restored it in some places, but were unable to finish before the Sienese invaded.
After more than half a millennium, the walls were finally and painstaking finished at the end of the last century.
The current walls are made of large sandstone blocks. Most are the original Roman laid ones. Along the walls are three doors, and what was once a post office - the earliest example of its kind in Italian history.
The 18 towers are still standing, although in various stages of decay. The ones that faced the sea were rectangular, while the three that didn't were circular.
The design of these walls was so clever that the Romans even had their own separate acropolis. It was located to the south on a hill and surrounded by its own independent walls, with a single access door from the inside.
A sewer system also wove its way in and out of the walls. It's an amazing feat of architecture and shows exactly how industrious the Romans were.