I use to live 3 minutes away from this ancient monument, a fascinating place to visit. Rome allows you such visits of sites that are to this day functioning. Via Nomentana in Rome starts at Porta Pia where the Aurelian Walls stand. At this point you are outside the city (in ancient time). The underground all along Via Nomentana is full of various catacombs and ancient places of worship. Unfortunately very few tourists are aware of the wealth of sites to visit and stick to the Roman Forum (the ancient centre). I find that sites like Santa Constanza are a lot easier to assimilate and understand because the iconography is closer to us and our modern traditions. It is also a site which shows that in terms of religion, Christianity did not start with Peter but much later as an established religion in the Fourth Century with Emperor Constantine, he in many ways was the enabler and politician who allowed Christianity to go from Cult to Religion.