The Mamertine Prison in Rome was located on the east side of the Capitoline Hill, adjacent to the Roman Forum, and near the Arch of Septimius Severus. According to early "church" tradition, both Peter and Paul were held prior to their execution in this small building consisting of only two cells, one above the other. Although its name is medieval, the prison was constructed in the 7th century B.C., and consisted of a vast network of dungeons under the city's main sewer.
It is probably this prison that Paul had reference to in 2 Timothy 4:21, when he urged Timothy to visit him before winter. Roman jails were subterranean cells used for holding political prisoners and criminals for short periods of time in cramped, miserable conditions. The practice of confining prisoners for extended periods of time as a form of punishment was not widespread until the 15th century A.D.