Short answers, pulled from the story.
Campus Martius was a publicly owned area of about 2 square kilometers used for military musters, elections, religious festivals, and eventually monumental construction. Roman men assembled there each spring before campaigns, citizens gathered for census counts every five years at the Villa Publica established in 435 BC, and the space hosted major festivals including the October Horse on the fifteenth of October and the Secular Games.
The Campus Martius was named after Mars, god of both war and agriculture, who was among the most significant figures in the Roman pantheon. It may have taken its name from the Ara Martis, an altar to Mars mentioned in sources going back to the eighth century BC. Paul W. Jacobs III links the site's significance to Mars's dual patronage of military and agricultural life.
Augustus and his circle transformed the Campus Martius with a series of politically charged buildings. Marcus Agrippa built the Pantheon, the Baths of Agrippa, and the Saepta Julia there on behalf of Augustus, and completed the Aqua Virgo aqueduct in 19 BC to supply the new baths. The Senate decreed the construction of the Ara Pacis, begun in 13 BCE and dedicated in 9 BCE, and the Mausoleum of Augustus occupied the northern part of the field.
The Ara Pacis is an altar built during the reign of Augustus, begun in 13 BCE and dedicated in 9 BCE on Livia's birthday. It stood in the Campus Martius and its four carved panels depicted Augustus's family, the Senate, the goddess Tellus, and a founding sacrifice, together conveying a message of peace and dynastic permanence.
After barbarian invasions cut the aqueducts supplying Rome's hilltop neighborhoods, residents moved to the Campus Martius to remain near the Tiber River as a water source. The area also benefited from its proximity to the Vatican, which drew a continuous stream of pilgrims who brought wealth to the district. By the Middle Ages it had become the most populous area of Rome.
The Pantheon, originally built by Marcus Agrippa and later rebuilt by Hadrian, survived the fall of the Roman Empire but was stripped of its bronze roof tiles in 663 AD, which were replaced with lead. A writer named Gregorius attributed this act to excessive greed. The structure was eventually given a new role rather than being razed, and still stands today.