The circular building located at the entrance of the Forum, which now houses a small archeological exhibition, was possibly built in the early 4th century as a Roman temple which may have been dedicated to Valerius Romulus, deified son of the emperor Maxentius; it is often referred to as the Temple of Romulus.

The Temple is traditionally held to have been dedicated by Emperor Maxentius to his son and co-consul Valerius Romulus, who died in 309 and was given divine honours.

The temple building was probably part of a rebuilding program of "incredible intensity" undertaken by Maxentius in the area, following a disastrous fire in 306; the project was only part-complete at his death.

The bodies of Saints Mark and Marcellian were translated, perhaps in the ninth century, to this church, where they were rediscovered in 1583 during the reign of Pope Gregory XIII.

The works, projected by Orazio Torriani and directed by Luigi Arrigucci, raised the floor level seven metres, bringing it equal with the Campo Vaccino, thus avoiding the infiltration of water.

The importance of this basilica for the history of medicine is not only related to the fact that the two brothers were physicians and were honoured as patron saints of physicians, surgeons, pharmacists and veterinarians, with veneration dating from the mid 5th century CE, but also to the tradition according to which Claudius Galen himself lectured in the Library of the Temple of Peace ("Bibliotheca Pacis").