Metropolis (メトロポリス,Metoroporisu?) is a location in Sonic Chronicles: The Dark Brotherhood. The game actually starts with the destruction of Eggman's Metropolis. G.U.N. attacks the city in full military force, while Sonic, Tails, Knuckles and Amy battle the Doctor on board his latest Egg Carrier. Tails succeeds in sabotaging the Carrier, such that the airship disintegrates and crashes into Metropolis, devastating the city in a massive explosion as the GUN forces and the heroes retreat.
Following this defeat and with Eggman missing and presumed dead, the ruins of Metropolis were left to rust. The only visitors were those human scavengers brave or foolhardy enough to raid the ghost city for salvageable metals and technology, until more guests began to arrive.
The "Marauders" secretively moved into the ruins of Metropolis, using the devastated city as their main base of operations on Sonic's world. The Marauders hacked into obsolete Eggman robots, deploying them to guard their operations while Procurator Shade moved forwards with the plan to capture the Chaos Emeralds. Using the Emeralds, the Marauders were able to move Angel Island from above the Mystic Ruins to Metropolis.
In Chapter 5, Eggman provisioned Sonic with the necessary information to infiltrate the Metropolis Underground and stop the Marauders. The hedgehog arrived seconds too late, though; after expropriating the Master Emerald from its plinth, the Marauders abandoned the stronghold and returned to the Twilight Cage—leaving Knuckles' floating continent to crash square into the city.
When the player enters Metropolis, they will no longer be able to go back to Blue Ridge Zone or any other previous location. Dr. Eggman will warn of this before leaving Blue Ridge Zone.
The same thing will happen with Metropolis once Tails' party takes the teleporter to Angel Island. Leaving before rescuing E-123 Omega will leave him locked for the rest of the game.
When the player finds Omega, if Shadow isn't in Tails' party, Tails will call Shadow and tell him they found Omega. However shortly after fighting robots that guard Omega, the following cutscene acts as if Shadow is in Tails party, regardless if the player left him in the Metropolis underground or put him in Sonic's party.
↑Later in The Dark Brotherhood, the team learns that Robotnik was in fact saved and nursed back to health by one of his medical robots; he is eventually discovered, slowly rebuilding his power base from a secret underground facility in Green Hill Zone.
↑As alleged by the adviser for the tower escalator puzzle, Sonic Chronicles: The Dark Brotherhood, Chapter 5