The forum was the administrative, political, and religious center of the city. It was a large, arcaded square around which the main public buildings were distributed. Under its pavement sat four large water cisterns. The remodeling of the forum took place under Emperors Tiberius, Caligula, and Claudio, ending roughly between the years 20 and 50.
The four cisterns under the center of the forum were covered by barrel vaults and are an exemplary example of Roman hydraulic engineering. Its walls are made of opus caementicium, the floor has a quarter-round molding at the juncture with the wall in order to facilitate cleaning and to prevent leaks. The water ran through lead pipes, also known as fistulae plumbae.