The martyrs of the Mamertine prison on the right, together with a list of saints who later visited the place.
From the Catholic Encyclopedia
A third Roman feast of the Apostles takes place on 1 August: the feast of St. Peter's Chains. This feast was originally the dedication feast of the church of the Apostle, erected on the Esquiline Hill in the fourth century. A titular priest of the church, Philippus, was papal legate at the Council of Ephesus in 431. The church was rebuilt by Sixtus III (432-40) at the expense of the Byzantine imperial family. Either the solemn consecration took place on 1 August, or this was the day of dedication of the earlier church. Perhaps this day was selected to replace the heathen festivities which took place on 1 August. In this church, which is still standing (S. Pietro in Vincoli), were probably preserved from the fourth century St. Peter's chains, which were greatly venerated, small filings from the chains being regarded as precious relics. The church thus early received the name in Vinculis, and the feast of 1 August became the feast of St. Peter's Chains (Duchesne, op. cit., 286 sqq.; Kellner, loc. cit., 216 sqq.). The memory of both Peter and Paul was later associated also with two places of ancient Rome: the Via Sacra, outside the Forum, where the magician Simon was said to have been hurled down at the prayer of Peter and the prison Tullianum, or Carcer Mamertinus, where the Apostles were supposed to have been kept until their execution. At both these places, also, shrines of the Apostles were erected, and that of the Mamertine Prison still remains in almost its original form from the early Roman time. These local commemorations of the Apostles are based on legends, and no special celebrations are held in the two churches. It is, however, not impossible that Peter and Paul were actually confined in the chief prison in Rome at the fort of the Capitol, of which the present Carcer Mamertinus is a remnant.