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 one identifies the little round temple as the Temple of Hercules and its neighbour, which had been thought to be the Temple of Fortuna Virilis, as the Temple of Portunus.
Model of Rome in the days of Constantine the Great
Museo della Civilta, Rome