The massive 1st-century AD monument is not only a porta (city gate), but part of the Acqua Claudia aqueduct. It gives you an idea of the grand scale of ancient Roman public works. On the Piazzale Labicano side of the portal, to the east, is the curious Baker's Tomb, erected in the 1st century BC by a prosperous baker, shaped like an oven to signal the deceased's trade. The site is especially accessible given that Rome's first tram depot (going back to 1889) is just nearby.
Junction of Via Eleniana, Via di Porta Maggiore, and Via Casilina, Rome, 00184, Italy