Fillydelphia was mentioned as once suffering from a Parasprite infestation, which required Princess Celestia's personal attention.
During the War
Fillydelphia became the manufacturing hub of Equestria during the war, a large number of the weapons and machines of wartime Equestria were produced here. Fillydelphia also housed hubs for the ministries, similar to other major cities like Canterlot and Manehattan.
The Last Day
Fillydelphia, like Canterlot was a target for long range megaspell bombardment. A large portion of the city was destroyed, creating a massive crater that dominates Fillydelphia and is still highly radioactive.
The city lays in ruins, but has been occupied by powerful factions like the Steel Rangers and Red Eye. Many of the factories have been restored to serve the purposes of Red Eye's Army, allowing him to arm a large host of soldiers with a variety of deadly weaponry. The restored factories are also used by Red Eye to print books and other essentials, Red Eye has established himself as the slave master of the wasteland, with Fillydelphia as the slaver capital.
Littlepip infiltrated Red Eye's operations in Fillydelphia to steal information for the Steel Rangers. Littlepip later escaped with another slave, Xenith, but was soon recaptured. She was freed afterwards by Red Eye in return for his second in command who had been captured by the rest of Littlepip's companions.
Fillydelphia was one of the final targets of the Enclave operation, Operation Cauterize which saw the deployment of a Thunderhead Carrier and a vast number of troops and Cloudships to battle Red Eye. Fillydelphia survived the initial onslaught and Red Eye's forces were mobilized under Stern after Red Eye's death. While the Enclave and Red Eyes remaining forces fought against one-another, a coalition made up of Applejack Rangers, Gawd and Blackwings' Talons, Alicorns, and Steel Rangers all underneath the command of Littlepip entered the fray and begun attacking both sides. Xenith infiltrated Fillydelphia and targeted the Red Eye command structure, while Ditzy Doo absorbed radiation from the Fillydelphia crater and performed a Sonic Rainboom to ground the Enclaves' Raptors over Fillydelphia, the Twilight Society uses the Celestia One Megaspell to further devastate the Enclave and Red Eyes' Army in the city. All of it was used as a diversion to keep the Enclaves' attention away from Neighvarro while Littlepip gained access to the S.P.P. Hub.
In Other Stories
Murky Number Seven
The ruined metropolis is explored in much more detail by the titular protagonist. Murky Number Seven also revisits some of the places that Littlepip went to on her brief stay in the slaver city. The story goes into many of the inner workings of Fillydelphia, including the hierarchy of the slavers and slaves, the founding of Red Eye's empire, the various locations & factories of the city too. The time before Red Eye is also briefly explored as is the wartime era and characters.
Behind the Scenes
The setting and role of Fillydelphia are most likley based off of The Pitt, a slaver city that is the main setting of the eponymous Fallout 3 add-on The Pitt.