Tucked away along the banks of the Tiber River is an unsuspecting quiet area where much of Rome was thriving around the period of Christ's time on earth and before. Rather than bore you with details/directions, the Eyewitness Rome Guide has an indepth street-by-street walking tour map in the Aventine section on page 200 complete with directions, photos and information that should certainly merit at least a couple of hours of your time. Also not to be missed are the nearby remains of San Nicola in Carcere and the Theater of Marcellus.
When wandering this area among the relics, try to imagine it as what it was - Rome's central market area where foods, livestock and goods were traded next to what was the city's port on the Tiber. When looking at the round Temple of Hercules and nearby Temple of Portunus, it doesn't seem fitting that this area could ever be abuzz with activity the way Campo de Fiori is with it's markets or Piazza Venezia with its immense ammount of traffic/people. Did those 2,000 years ago see these structures as anything significant beyond a temple, or were they mere buildings blended into their daily lives...like we so often take for granted our familiar surroundings?
San Nicola and the Theatre of Marcellus at first look like more of the same - crumbling ancient structures so questionable as to their safety, they've been fenced off from public viewing. But next to Marcellus, which from the outside looks something like a mini-Colosseum, are three lonely columns indicating the significance of what was...and still is. Standing since circa 431 B.C., the columns are all that remain from a Temple of Apollo; compared to the theatre which was begun by Julius Caesar and finished by Augusta. This site, recognized as one of Rome's three permanent theatres, could hold up to 20,000 people. Today, guidebooks say the interior is all but missing and the outer structure has been heavily altered over the centuries to convert the theatre into fortresses, residences and other things.
Another separate area too valuable to miss is the Largo Argentina Sacred Area (Via Arenula) which contains foundational remains of what's believed to be the oldest temples found so far in all of Rome. The four places of worship dated back to early 3rd century B.C. and the Roman Republic. They're sunken very low in the ground and observed from street level. When passing, you're likely to think it's just more piles of rubble - which it is! A valuable tip for those who likely won't know the "main attraction": behind remains of Temples B & C are foundational tufa blocks from a rectangular building which housed where the Roman senate met. It's also where Julius Caesar was assassinated!