Mamertine Prison (Rome)

 chapel, prison, religion, Roman Empire

The Mamertine Prison (also referred to as the Tullianum) was a prison (carcer) located in the Forum Romanum 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 name "Mamertine" is medieval in origin, and may be a reference to a nearby temple of Mars, or to the legend of St. Peter, whom tradition holds to have been imprisoned there before his martyrdom: the legend states that Saints Martinian and Processus, supposedly his warders, were converted and baptized by Peter before being martyred themselves. (Biblical account Acts Chapter 12; 3-19)
The reason the cross is inverted is due to Saint Peters being Crucified upside down.