The Mamertine Prison (Italian Carcere Mamertino), in antiquity the Tullianum, was a prison (carcer) located in the Comitium in ancient Rome. It was located on the northeastern slope of the Capitoline Hill, facing the Curia and the imperial fora of Nerva, Vespasian, and Augustus. Located between it and the Tabularium (record house) was a flight of stairs leading to the Arx of the Capitoline known as the Gemonian stairs.
The church of San Giuseppe dei Falegnami now stands above the Mamertine.(Wikipedia)