From Sonic Retro
Robotropolis is a city which appears in the Sonic the Hedgehog cartoon series (known as "SatAM" by fans), the Sonic the Hedgehog comic book series published by Archie Comics, and the Sonic Underground cartoon. A city in the English dubbed version of Sonic the Hedgehog: The Movie is also referred to as Robotropolis.
Sonic the Hedgehog cartoon and Archie comics
The city was orignally known as Mobotropolis and was once been ruled by the House of Acorn, until King Acorn had been dethroned by his treacherous War Minister, Dr. Robotnik. The king was sentenced to exile in the Zone of Silence. As soon as Dr Robotnik took over, he roboticized roughly 85% of the city's population, the remaining 15% had escaped. The city was then rechristened Robotropolis.
Robotropolis was the seat of Robotnik's power, acting as his empire's capital city. After numerous engagements against the Knothole Freedom Fighters, Robotnik was eventually killed by his own weapon, the Ultimate Annihilator (due to tampering from his nephew Snively). The Mobians then retook the city and began trying to restore the city to its former glory. However, an alternate universe version of Robotnik arrived and was able to retake the city with relative ease. He distracted the Freedom Fighters by leading them to his satellites in orbit, while launching hordes of Shadowbots to reclaim the city. He then rechristened the city Robotropolis once again.
Robotropolis was later destroyed in a nuclear strike by Station Square. The harmful fallout is currently kept within the former site of the city by a shield originally designed to protect the city.
In the miniseries Mobius: 25 Years Later, Mobotroplis appears to have been retaken and rebuilt, as Rotor the Walrus was seen disembarking from a ship that had departed from Mobotropolis when he arrived on Portal, Angel Island.
Mobotropolis was later reborn when the 'city' created by A.D.A.M.'s nanites was transformed into a exact recreation of it and then christened 'New Mobotropolis'.
New Mobotropolis is a completely computerized city which has a defense system revolved mainly around energy shields. The city also has teleportation technology, which can be used to send individuals from one place to another. Presumably, as the controlling intelligence of the city, NICOLE also has virtually unlimited power within its boundaries.
Following his campaign on Angel Island, the new Enerjak made his way to New Mobotropolis with the intent of ridding it of all technology. He was stopped in this objective by the arrival of first Sonic and then Shadow the Hedgehog, whom engaged him in a battle that left the city badly damaged. Enerjak was then captured by the Egg Fleet, leaving the inhabitants of New Mobotropolis to begin repairs.
In series Sonic Underground, the former ruler of Mobotropolis had been Sonic's mother, Queen Aleena Hedgehog. The city in this cartoon was attacked and conquered by Robotnik, and the queen fled the city. Sonic himself, along with his siblings Manic and Sonia, were heirs to the throne, and fated to one day form a council of four with their mother and take back control of Mobius.
In look and design, Robotropolis in Sonic Underground is exactly like it was in "SatAM"; however, the Robotropolis in Sonic Underground is still inhabited by Mobians, possessing a run-down slum for the underclasses and more upscale areas for the aristocratic elite.
In Sonic the Hedgehog: The Movie, Robotropolis is the massive techno-city constructed by Robotnik in the Land of Darkness and is near or is in a post-apocalyptic city which resembles New York City, called Ancient Ruins. In the original Japanese version Mobotropolis is not referred to and the name for the city is Eggmanland. It was changed to Robotropolis in the English dubbed version to tie it in with the American continuity.