Located in the heart of the Roman Forum, beneath the church of San Giuseppe dei Falegnami, the Mamertine Prison (or Tullianum) is the oldest prison in Rome.
For centuries, it served as a maximum-security facility for enemies of the state awaiting execution; the Roman legal system, in fact, did not provide for imprisonment as a punishment, but only fines, exile, or death. Its strategic location at the foot of the Capitoline Hill made it a stern reminder of the city's justice.

The structure is divided into two levels: the lower level, the Tullianum, dates back to the Archaic period (8th-7th centuries BC), while the upper level, the Carcer, dates back to the Republican era. According to Christian tradition, Saints Peter and Paul spent their last days here.
Legend has it that Saint Peter, descending into the Tullianum, struck his head, leaving a mark in the rock, and miraculously caused a spring of water to flow to baptize his jailers, Processo and Martiniano. Transformed into a place of worship since the 4th century, the site is now an integral part of the monumental complex dedicated to Saint Peter in Prison.