Mobotropolis was first created hundreds of years ago, after the Mobian Alexander decided that people could benefit from the combined efforts of a variety of different species. After it's completion, the citizens make Alexander their king, as he was ultimately responsible for their communion and the city's construction.
Due to the Mobian's philosophy to preserve nature, the Kingdom of Acorn and the city of Mobotropolis' technological development quickly stagnated and remained in the pre-industrial and medival state. This changed dramatically when King Frederick Acorn took in the exiled Overlander scientist Nate Morgan. Nate had been working on the development of the Power Rings - a clean, alternative fuel supply to replace the toxic fossil fuels frequently used by his people. Nate complated creation the Power Rings while in Mobotrpolis and, with a clean power supply, Mobotrpolis was re-created into a modern city.
Following Frederick stepping down and his son Maximillian Acorn assuming the mantle as king, Mobotrpopolis suffered a major setback. The current Warlord Kodos and the Royal Wizard Ixis Naugus felt threatened by the potential of Nate Morgan, and thus staged a conflict between the Overlanders and Kingdom of Acorn, blaming Nate for it. After Nate imposed self-exile, skermishes between the Mobians and Overlanders continued to persist though remained relatively small, until Kodos had the corpse of an Overlander scout sent back to their territory, and a corpse of one of his own kind's sent back to Mobotropolis, both with a note saying "this is what happens to all who oppose us". Both believed they were dealing with savages, and thus the Great War began.
Because the Mobians' refusal to use gun-like weapons (due to the The Paladin Incident in which Prince Emerson was accidentally killed by a pistol) the Overlanders had a strong military advantage. Mobotrpolis came under siege of mortar rounds within the first year of the war, with many of its structures being destroyed. The tides of the war changed in the Mobian's favour after the Overlander Julian Kintobor defected and began upgrading the Mobians arsenal to use more modern weapons. After the Great War's end with the Mobian's victorious, Mobotropolis was repaired.
Shortly after the Great War ended, the kingdom's Warlord, Julian Kintobor, took over the city using his hordes of SWATbots. The majority of the city's population was captured and roboticized, though some managed to escape to the sanctuary of Knothole in the Great Forest, including the members of the Original Freedom Fighters, King Maximillian Acorn's daughter Sally and a few other children which went on to become the Knothole Freedom Fighters. The city was subsequently rechristened Robotropolis, and became the capital of Dr. Ivo Robotnik's global empire.
There have been many attempts to retake the city over the next decade. Most left the city standing for the most part. However, when an earthquake threatened to level the city, Robotnik temporarily retreated to a flying fortress with his nephew Snively and as many of the SWATbots as the ship could carry. Sally, Sonic the Hedgehog, and Sonic's Uncle Chuck used the time to deroboticize as many Robians as possible, and barely escaped the city. When Robotnik returned, he planned on using a forcefield to keep any intruders out while the city was being repaired, but Snively's latest failure ruined the force field.
Eventually, Robotnik managed to rebuild the city, before a greivous blunder was committed on his part. He had not properly safeguarded a supply of nuclear warheads, and had to retreat to a bomb shelter when he saw Mecha Sonic and Mecha Knuckles about to crash into the city and set off the bombs. The damage was even more severe than before, and reconstruction was further delayed when Sonic sent Nack the Weasel's ion-powered sky-sled on auto-pilot and set it to self destruct, leading to yet another major explosion in Robotropolis that could be felt from Knothole villiage.
Of course, as far as bad ideas go, putting Snively in charge of reconstruction was one of the worst. As soon as Robotnik took off for the Death Egg, Snively ordered all of the SWATbot's to stop reconstruction and give him all of the perks (a massage, a snappy uniform, a hot oil bath, bon-bons, etc.) and had the city renamed Snivopolis. Naturally, Robotnik stopped that once he returned from the now-destroyed Death Egg.
After numerous engagements against the Knothole Freedom Fighters, Robotnik was eventually killed by his own weapon, the Ultimate Annihilator (due to tampering from Snively). The Mobians then retook the city and began the tedious task of restoring the city to its former glory.
However, the arrival of an alternate universe version of Robotnik (now calling himself Dr. Eggman) spelled disaster for Mobotropolis. Dr. Eggman was able to retake the city with relative ease, distracting the Freedom Fighters by leading them to his satellites in orbit, while launching hordes of Shadowbots to reclaim the city. When the Mobians fled to Knothole, Eggman rechristened the city Robotropolis.
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. Fort Acorn exists just outside of the city to protect the force field against attackers.
The Dome over Robotropolis