Happy, you would say, were the forbears of our great-grandfathers, happy the days of old which under Kings and Tribunes beheld Rome satisfied with a single gaol! Juvenal Satire III lines 312-3 translated by G. G. Ramsay
The one jail was traditionally supposed to have been built by Ancus Martius, the fourth king of Rome, with an underground dungeon added by Servius Tullius, the sixth king of Rome. It is here that Venus in Copper opens with Marcus Didius Falco having fallen victim to the wiles of his arch-enemy, Anacrites. Imprisonment of citizens does not seem to have been used as a punishment in Ancient Rome in Falco's day. The main function of the jail was as a place to keep state prisoners awaiting execution and people accused of crimes but unable to provide a surety to guarantee their appearance when their trial came up.
The Latin word for prison is carcer. The underground dungeon was called the Tullianum. This is the reason it was supposed to have been built by Servius Tullius, though it could also be derived from the word tullus, meaning a spring, some people thinking it was originally built as a reservoir. Varro, a Roman scholar of the first century BC, informs us that the Tullianum was also called the Lautumiae or quarries, either because there used to be a quarry nearby or after the quarries in Syracuse also used a place of detention. After our period, the building became known as the Mamertine Prison, which was and is revered as the place where Saint Peter was imprisoned in Rome.
For further information and links to the sources, see the entries for carcer in Smith's Dictionary of Greek and Roman Antiquities and Platner's Topographical Dictionary of Ancient Rome. LacusCurtius also has an an Italian translation for the entry carcer from Huelsen's Das Forum Romanum.