Short answers, pulled from the story.
King Romulus ascended to the afterlife during a storm cloud event in the seventh century B.C. This land situated between Rome and the Tiber River later became property of Tarquinius Superbus before being dedicated to Mars following his exile.
In 435 B.C., officials established the Villa Publica within a prepared 300-meter clearing to serve as a gathering space for citizens counted every five years in a census. No permanent structures existed there at that time, and no additions would be made for another two centuries.
Marcus Agrippa built the Pantheon on the Campus Martius which Hadrian later rebuilt as it still stands today. The area also included the Mausoleum of Augustus occupying the non-populated northern field and the Theatre of Marcellus constructed around Caligula's time.
The Ara Pacis altar was begun in 13 BCE under Augustus and dedicated in 9 BCE on Livia's birthday. The south panel depicted a religious procession with Augustus, Agrippa, Livia, Tiberius, and other family members standing together while the north panel showed the senate in a procession alongside Augustus rather than against him.
In 1555, Pope Paul IV designated southern parts of the Campus Martius as the ghetto containing the city's Jewish population. This designation occurred after Muslim Saracen pirates raided coastal towns along the Via Aurelia and when malaria spread made the route unsafe passing through unhealthy marshes near coastal lakes like Orbetello lagoon.