Originally, they were thermal baths dating back to the 1st – 2nd century A.D. but after a period of inactivity of about two centuries, in the 4th century they were transformed into the Church of Santa Maria, with an attached large cemetery.
The name was probably due to the architectural structure: it features a round area enclosed within a quadrilateral topped by a dome. In 1943 it was hit by bombs and only recently, in the ‘40s and ‘50s, the original structure was recovered.