Romans did not have prisons that relate to how we think of them in the modern world. Accused wealthy citizens were simply kept under house arrest, provided they behaved, until a trial could take place.
The poor generally found justice swift and usually fatal. Outside of the cities, a villa might have three areas to keep slaves, one for those who were well behaved, one for those to keep shackled and one for those allowed a bit more freedom.
Actual prisons in Rome truly served as a holding place for those condemned to die. Occasionally the accused might be detained to await trial, but usually those awaiting trial were encouraged to go into voluntary exile. Those awaiting trial were called "carcer" or "publica vincula."
The most famous Roman prison can still be visited today. It is located just outside the Forum Romanum buried at the foot of the Capitoline Hill. It was Ancus Marcius, the fourth king of Rome, who, sometime during his reign (640-616 BC) constructed this dark, damp and foreboding subterranean structure.
One enters the prison today by following steps down from the Capitoline. Looking ahead one sees, on a sunny day, the remains of the glistening white marble of the Forum. By contrast, a turn to the left and down a few more stairs finds the visitor at the entrance to the prison. It is a small room, with a hole in the floor. This was the entrance to the dungeon, constructed by the orders of the 6th king of Rome, Servius Tullius.
Sallust described it as about twelve feet deep into the ground. "Its appearance is disgusting and vile by reason of the filth, the darkness and the stench." It was into this room, 6 1/2 ft. high, thirty feet long and twenty-two feet wide, that prisoners who had been condemned to die either by strangulation or starvation were thrown. One attributes the phrase "to be cast into prison" had its origins here.
Even today one can see an iron door which opens to the Cloaca Maxima, then the main sewer of Rome which emptied into the Tiber. It is said that the dead were cast away through this door. Sometimes, the dead were displayed on the marble stairs before being sent into the Tiber.
Today one can visit the dungeon via a narrow staircase. It seems even smaller than the above mentioned size because there are so many visitors to this eerie dungeon. Among the famous who spent their last days here were the leader of the Gauls, Vercingetorix who had tried to rally the Gallic tribes into one union against Caesar and, obviously, did not meet success, Simon Bar Jioras, the defender of Jerusalem defeated by Titus in the sack of the city in A.D. 70, and St Paul.