The 17th century church of San Giuseppe dei Falegnami (St Joseph of the Carpenters) stands above the ancient Mamertine Prison. According to legend, St Peter and St Paul were once held in the Mamertine prison. There they converted and baptised, not only a number of their fellow prisoners, but also their jailers Processus and Martinian, who, in turn, became martyrs and saints.
The name 'Mamertine' is medieval; in antiquity the prison was known as the Tullianum. Imprisonment was not a sentence under Roman Law; incarceration in facilities such as the Tullianum was only intended to be a temporary measure prior to trial or execution. The prisoners incarcerated in the Tullianum were generally figures of importance and include Jugurtha (c. 160-104 BCE)), king of Numidia, the Gallic chief Vercingetorix (82 - 46 BCE), and Simon bar Giora, a Jewish revolutionary who was executed in 70 CE.
A small altar in the prison commemorates the presence of St Peter. The small cross on the altar is inverted, since, according to tradition, Peter chose to be crucified upside down.