For all its high drama and dynastic strife, it was actually unusual in ancient Rome for anyone to spend much time in jail. High status prisoners awaiting trial were usually put up in various levels of comfort in private homes. Low status prisoners never got trials. And even after conviction the Roman state did not have the concept of locking people up behind bars. You might be exiled. You might be executed. You might be sent to toil in the mines or as a galley slave. It seems to our later, more modern eyes to be a bit harsh although there is a certain austere economy to the system. You don't cost the state money before your trial. Or after it.
But there was a small role in Rome for what was called a Carcer, a temporary place of confinement from which our later and more inclusive word "incarceration" descends.
Rome's most famous ancient lock up of course is the Mamertine Prison, sometimes called The Tullianum. Neither name is fully understood, the former is of Medieval date and might refer to a nearby and now lost Temple of Mars. The latter is a reference to one or another of the late Kings of Rome who had similar names. But whether they or someone else actually built the place is unclear.
In any event it was built early, perhaps 6th Century BC, and held some famous prisoners over the centuries. Sejaunus, the scheming Praetorian Prefect (memorably played by Patrick Stewart!) was held here before being strangled and his body torn to bits by a mob on the nearby Forum. Vercingetorix the Gaul was atypically held here for five years before his execution. But that was less of a sentence than a waiting period...he had to be displayed in a Triumph to Caesar before being eliminated.
There is actually an upper and a lower room to the Mamertine Prison. Captives were lowered down into these, doors you see below were for the benefit of later pilgrims. The round grate in the floor was the original entrance to the lower chamber. Prisoners were just thrown down to the lower cell.
A bit of graffiti.
The lower room probably started out as a water cistern. It is a claustrophobic place to be sure.
The column supposedly is where Saints Peter and Paul were chained during their traditional but unconfirmed stays in the Mamertine Prison. There is also a "miraculous" spring that they used to baptize their fellow prisoners.
I assumed this outer door depicted Saint Peter, but he does not have the traditional keys that would identify him. He does have a sword, perhaps recalling his defense of Christ in the Garden of Gethsemane.
According to this sign in the lower chamber, you can still see the dent in the rock where Saint Peter's head hit when he was thrown down!
There were some other interesting sites that I didn't get decent photos of. An altar with an upside down cross for instance, also recalling Saint Peter. But because the site is so small it does not take long to visit. I entered by an entrance off of the long stairs coming up from the Forum to the Capitoline Hill. It is not well marked, so you have to go out of your way to find it. I think there is also access from the church built over the Carcer. It is San Giuseppe dei Falagnami , Saint Joseph's of the Carpenters. A small admission fee is charged.