Central City, previously known as Station Square (ステーションスクエア, Sutēshon Sukuea?), is the capitol of the United Federation. It is a heavily populated city famous for its many attractions including an amusement park, a hotel/restaurant, and a casino district. A train station in the city allows travel to the Mystic Ruins. It has a human population of 2,856,427.
Main article: Perfect Chaos incident
In the beginning of Sonic Adventure, Sonic goes to Central City, known then as Station Square, for fun, and stumbles across Chaos, a strange creature that is wreaking havoc on the city. Sonic defeats it, and later learns that Dr. Eggman had released Chaos and is planning on collecting the seven Chaos Emeralds to transform Chaos into an incredibly powerful beast that can destroy Station Square, so that Eggman can build his Eggmanland on its ruins.
Sonic and Tails go after the Chaos Emeralds in order to stop Chaos from obtaining them, and one was found in Station Square (in Casinopolis, to be precise). Station Square also plays a role in the other characters' stories: Knuckles finds three pieces of the Master Emerald there, Amy lives there, and Big and Gamma both go there looking for Froggy.
Eventually the heroes face off against Eggman in his Egg Carrier, defeating him and Chaos. In a fit of rage, Eggman fires a gigantic missile into the heart of the city. However, the rocket misfires and turns out to be a dud. Tails races Dr. Eggman to the missile and manages to deactivate it completely before the Doctor gets a chance to set it off manually.
However, Chaos eventually obtains all seven Chaos Emeralds, and transforms into Perfect Chaos. Perfect Chaos easily destroys Station Square by flooding the city in an enormous tsunami, and proceeds to roam around the rubble of buildings submerged in the water. Super Sonic ultimately defeats the beast and leaves the destroyed city in pursuit of a fleeing Eggman.
After these events, the people of Station Square worked together to rebuild the settlement, eventually renaming it Central City. The United Federation named this city its capitol, and the White House was erected to act as a "home away from home" for the United Federation's president.
Sonic Adventure 2
Main article: Space Colony ARK incident
The events of Sonic Adventure 2 take place in the newly rebuilt Central City about five years after the Perfect Chaos incident. Dr. Eggman awakens Shadow at a G.U.N. island, and the two resolve to collect the Chaos Emeralds to power the Eclipse Cannon on the Space Colony ARK. Shadow runs through Radical Highway to steal a Chaos Emerald from a local bank, and he is mistaken by the locals for Sonic, who is taken into custody. The blue hedgehog manages to escape, however, and escapes through the city.
The city is thrown into turmoil once again when Eggman blew up the moon using the Eclipse Cannon as a threat to the United Federation to give in to the doctor. Knuckles enters the Aquatic Mine as part of his search for the Master Emerald shards. He then proceeds to exit through the city's sewer system and meets up with Tails and Amy. Tails then chases the United Federation's president on Route 101.
When the Space Colony ARK is teleported back to its proper orbit, the city rejoiced in the heroes' victory.
Central City is the main setting in Sonic Battle. In it, there's an apartment where Amy Rose and Cream the Rabbit live, a skyscraper (called Central Lab) with a library and a famous database, and a highway called Battle Highway. Tails goes here to find out more about Emerl. Another part of Central City named Night Babylon also appears as a casino district where Rouge owns her own club and Dr. Eggman has one of his numerous bases.
Shadow the Hedgehog
Main article: Black Arms invasion
Shadow eventually turned against the Black Arms and ended the invasion by blowing up the Black Comet with the Eclipse Cannon, saving the planet.
Main article: Time Eater incident