The Forum Boarium was the open space close to the Tiber which was home to Rome's large meat and fish market. The two small temples standing in this area are the best preserved temples from Rome's republican period (from 2nd century BC). Their survival is largely down to the fact that they were consecrated as Christian churches in medieval times.
For a long time it was assumed that the little circular temple was in fact the Temple of Vesta. Today however it is identified as a Temple of Hercules and its neighbor, which had been thought to be the Temple of Fortuna Virilis, is known to be a Temple of Portunus.
Model of Rome in the days of Constantine the Great -- Museo della Civilta, Rome
Above -- "Arch of Janus" (so called, but the actual dedication is unknown)
Although originally dedicated to Septimius and both of his sons, the
image and name of Geta were obliterated, probably immediately after
Caracalla murdered him.