This church, St Peter in Chains, which dates back to the 5th century was built by the Empress Eudoxia to house the chains that had been used on St Peter. Legend narrates how two different sets of chains, one coming from Peter's prison in Jerusalem and the other from Rome's Mamertine Prison, were miraculously fused together during a papal celebration.

They can be seen in the glass case under the high altar. The unusual façade of the building comprises a 15th-century portico while the interior still conserves the splendid Doric columns of the original construction, despite the profound changes made during the course of the centuries.
This type of column is only rarely found in Roman architecture and it would be extremely difficult to discover from which ancient monument these columns were taken. Michelangelo placed his monument to Pope Julius II in the right-hand transept. It is commonly known simply as the Moses.