The Temple of Hercules Victor

The Temple of Hercules Victor is the oldest preserved building in Rome.  It is a small circular temple that was believed to have been built in the 2nd century BC.  It was dedicated to Hercules Victor, who was the protector of traders in the area.  It is across the street from the Mouth of Truth in the Basilica di Santa Maria in Cosmedin.  You cannot go inside because it is all gated up, but you can view it from the outside at any time.  It is located at Piazza della Bocca della Verita, 00186, in Rome.  

The temple is right next to a rectangular temple, the Temple of Portunus.  It was built in the 2nd century BC and is dedicated to the God Portunus.  It is only 36 by 62 feet.  It is also closed to the public and can only be viewed from the outside.