he Tullianum is a jail that consists of two rooms above each other. The top room is called the Carcer. It has the shape of a trapezium and is about five metres long and five metres wide and high. The walls and also the vault consist of blocks of tuff-stone. This room originally served as a stone quarry. The second room, the bottom one, is called Tullianum.
Ancus Marcius should have built the jail. The word 'Tullianum' refers to Tullus Hostillius or Servius Tullius. Writers like Varro, Sallust and Livy mention Servius Tullius as the creator of the state jail. It is also possible, that it comes from the word 'tullius', or tullus, which means 'source' or 'jet of water'. In that case this room has developed from a natural source. We can also think of a natural cavity, which was used as a well and water supply for the people. The shape also makes one think of an Etruscan dome-shaped grave. Modern research links up the rise of the Tullianum with the Gaul invasion in 386 BC.
The dome of the first floor room was removed with the installation of the upper room, which is also made of tuff-stone. Near the middle of the dome is a round opening of 70 centimetres. In Antiquity this was the only entrance to the room at the bottom.
The upper room, the Carcer, was the room where prisoners awaited their trials. The Tullianum was the place of execution. This jail gave room to the most famous enemies of Rome. Jugurtha from North Africa, Vercingetorix from Gaul, the brothers Gracchus and the accomplices of Catilina from Rome. According to a sixteenth century tradition, also the apostles Peter and Paul belong to the unhappy ones whose fate took them to this musty jail. A lattice work in front of a cavity in the wall marks the place where Peter's head was battered against the wall. According to a less hard-hearted tradition, it is the stone on which he rested his head.
The Tullianum is also known as the 'Mamertine Prison'. This name refers to the upper room with its shape of a trapezium. It dates from the Middle Ages, and possibly refers to a private owner of the place at that time.
Nowadays a chapel is situated above the Mamertine Prison, the Capella San Pietro in Carcere, the Chapel of St. Peter in Prison.