It is unknown when Pilvros was constructed but it was likely before the Human-Covenant war.
In 2558, one of the city's reactors began to destabilize at the Municipal Support Facility in the sublevels of Pilvros. The city was evacuated but people still lingered within city. A team was sent into the reactor facility to get War Games scans to hopefully analyze what happened and figure out the cause of the meltdown.
By 2558 it was a flourishing metropolis with an industry-focused working class and an economy-focused upper class. As an industrial powerhouse, Pilvros likely contributed greatly during the Great War.
The city had an extensive road network for trucks and other vehicles that were not flying. There also seems to even be some sort of inverse MagLev Train or monorail system in the bottom levels of the city. It also appears to have just barely enough room for two way flying car traffic through the city.