Metropolis

Metropolis is the central stronghold of Doctor Eggman in the video games and other media, such as the Sonic the Comic and Archie Comics continuities. This gargantuan machine-city is a polluted nightmare of steel and concrete, replete with robot sentries, Orwellian tele-screens, and free-flowing rivers of radioactive slurry. Many of the other Sonic continuities site the Doctor's base of operations in a very similar city-fortress—Archie, SatAM, Sonic Underground, and the Sonic OVA Movie all used Robotropolis.

For sixteen years (1992-2008), Metropolis occupied a sort of conceptual limbo in the game continuity. Although (as mentioned above) Megao/Metro/Robo/Mobotropolis functioned as Eggman's base of operations in most other Sonic media, within the games there was only the Metropolis Zone of Sonic the Hedgehog 2, which scarcely looked like a city, let alone the capital of the Eggman Empire. For over a decade, throughout the main series and spin-off games, Robotnik was shown operating out of mobile command centers or hidden research labs—with little to indicate that the Doctor still had a central stronghold. But a definitive answer finally came within Sonic Chronicles: The Dark Brotherhood, which revived the fortress-city as his base of operations.

Sonic the Hedgehog 2

 * Main article: Metropolis Zone (Sonic the Hedgehog 2)

Metropolis first appeared as Metropolis Zone, the eighth zone of Sonic the Hedgehog 2; coming after Oil Ocean Zone and preceding Sky Chase Zone. Unlike the previous seven levels, Metropolis contains three full Acts, and the stages are particularly massive—in some cases infinite, for in Act 2, the bottom of the level loops to the top! The boss of this zone is Flying Eggman.

Sonic Chronicles: The Dark Brotherhood

 * Main article: Metropolis (Sonic Chronicles: The Dark Brotherhood)

Sonic Chronicles: The Dark Brotherhood actually starts with the destruction of Eggman's Metropolis. GUN attacks the city in full military force, while Sonic, Tails,Knuckles and Amy battle the Doctor on board his latest Egg Carrier (which bears closest resemblance to the flagship from the end of Sonic Heroes). 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 escape from the scenery.

Following this defeat, and with Dr. 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.

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.

In the end sequence of The Dark Brotherhood, the team return from the Twilight Cage to find Metropolis entirely rebuilt - and bristling with weaponry. Eggman promptly shoots the Blue Cyclone out of the sky in something of a cliffhanger ending.

Sonic the Comic
In the continuity of Fleetway's Sonic the Comic, Doctor Robotnik managed to conquer all of the planet Mobius and establish a totalitarian regime when Sonic and his crew were catapulted six months into the future by the Omni-Viewer. He remained in power from Sonic the Comic #8 until #100, ruling the planet first from the Special Zone's Egg Fortress and later from Citadel Robotnik in the Metropolis City Zone. Metropolis was still inhabited by flesh-and-blood Mobians, even under Robotnik's rule; but the city was a dystopian police state, its residents living in perpetual fear of arbitrary arrest by Robotnik's Troopers and deportation to the Badnik Processing Plants.

Notably, this version of Metropolis existed before Robotnik came to power. The young resident known as Smokey remembered swimming at the East Metropolis Leisure Centre in the days before Robotnik, when Metropolis was still a fun place to live.

Metropolis City was a focal point for many of Robotnik's traps and surprises for Sonic (including the infamous Hero of the Year Award, and the ridiculous Mister Blobnik). Sonic and the gang visited the Zone quite frequently, attempting to disrupt Robotnik's rule there.

After the Doctor's regime was toppled in Issue #100 (thanks to an electromagnetic pulse generated by Super Sonic), the oppressed residents of Metropolis took back the city and razed Citadel Robotnik.

Archie Comics
Metropolis Zone has been featured in Archie's Sonic the Hedgehog comics during the Genesis arc, as Eggman resets time and space by using blue Chaos Emerald to power up Death Egg Mark 2. Metropolis Zone may seem to being part with Oil Ocean Zone as Eggman's temporary base. In Sonic the Hedgehog#228, Sonic jumps from the Tornado to inside of Metropolis and in the next issue, he proceeds his mission through the zone.

He encounters all badniks and uses many gimmicks of the zone until he reaches to the room, where Eggman has been waiting for him. Until Sonic starts memorizing events before reseting, Eggman sends his fake replicas from the rotating balloons to after Sonic. As the other Freedom Fighters manages to shut down electric power off from Eggman's base, the doctor himself then escapes with his Egg Mobile to the Wing Fortress Zone as Sonic also follows him too with Tails, who's riding the Tornado.

In Sonic the Hedgehog #255, Sonic and Tails infiltrate Metropolis Zone once more, this time to find and rescue Bunnie Rabbot-D'Coolette. The duo jump from the Tornado and skydive into the city, but on their way up the control tower encounter an Egg Soldier and the Foreman, who lock themselves in the command room with Sonic and Tails. The Egg Soldier, however, is Bunnie in disguise, and the rabbit dispatches the other guard with a swift uppercut. Sonic, Tails, and Bunnie fight their way out of Metropolis, escaping on the Tornado.

Trivia

 * In Sonic Chronicles: The Dark Brotherhood, the map of Metropolis has a Sega Genesis, the Eggrobo from Sonic & Knuckles, the Drill Mobile from Marble Garden Zone of Sonic the Hedgehog 3, and the Egg Capsule from the games from Sonic the Hedgehog 2 to Sonic & Knuckles.
 * The music of Metropolis in Sonic Chronicles: The Dark Brotherhood is a remix of Panic Puppet Zone Act 1 from the Sega Saturn soundtrack of Sonic 3D Blast.
 * In Sonic the Hedgehog 2, the Metropolis was clearly located on West Side Island; in Sonic Chronicles: The Dark Brotherhood, it appears to be part of the same landmass as other locations in Sonic's world, implying that this version of the city is somewhere on South Island.