Empire City is a high-rise metropolis based off the hustling and bustling city of New York City, also retaining features from other famous cities in the U.S. such as the fountain in the middle of the hub world that bears a resemblance to the Buckingham Fountain in Chicago. Endless highways, mile-high skyscrapers, and urban surroundings characterize Empire City, with bridges similar to the George Washington Bridge and the Verrazano Bridge as well as tunnels similar to the Lincoln Tunnel making it a near mirror image to the Big Apple.
During this crisis, Empire City was threatened to be overrun by Dark Gaia's Minions. Amidst this chaos, however, Sonic the Werehog arrived in Empire City with Chip and began clearing out Dark Gaia's Minions in Skyscraper Scamper which forced the monsters to retreat and ensured Empire City's safety for the time being.
A short while later, Eggman decided to make his move and had his robot army invade Empire City. Fortunately, once hearing rapports about Badniks in Empire City, Sonic and Chip returned to Empire City, where Sonic managed to drive off the invading forces, saving Empire City once more.
Some time later, Empire City and its continent were returned to their proper place when Sonic and Chip used the Gaia Temple in Shamar and the Chaos Emeralds to restore their fragment of the planet.
Empire City appears to be based on different cities in the U.S like Chicago, Seattle and New York City, especially considering the fact that New York City is nicknamed "The Empire City" after New York State's nickname of "The Empire State." Also, the flag looks much like the United States' flag, except the current U.S. flag has fifty stars while the one for Empire City has only twelve, the U.S. flag has thirteen stripes while the Empire City flag has seventeen and the blue area covers an entire side of the Empire City flag while the U.S. flag only has a corner covered by blue.
This is the only location in Sonic Unleashed to not appear in the Wii/PS2 version.
Because they both have locations based upon New York City, it is possible that Empire City is another part of Station Square from Sonic Adventure. Furthermore, due to the design of Empire City's flag originating from the United States' flag, one could speculate that Empire City is part of the United Federation.
Also, certain parts of the acts share a slight resemblance to Crisis City from Sonic the Hedgehog (2006). For one, they both include giant skyscrapers, there is a loop similar to one in Crisis City, and they both include seemingly endless highways. (The one in Crisis City has Sonic outrunning a tornado, and the highway in Empire City is guarded by an Interceptor.)
Empire City is one of the two locations in Sonic Unleashed, which does not have a Gaia Temple, the other being Apotos.
Empire City bears a close resemblance to Crisis City, a stage in Sonic the Hedgehog (2006), although it is not destroyed. If you look very carefully at the billboards, they are exactly the same as the ones seen in Crisis City, not to mention similar architecture.