The Robotnik Empire is a political body that appears in Sonic Underground. It is a totalitarian regime ruled by the infamous Dr. Robotnik. It was created on Mobius shortly after Robotnik's takeover of the planet, with its capital city being Robotropolis. Its main enemies are the resistance and their numerous allies. Regardless, the Robotnik Empire continues to encompass much of Mobius.
Just after Queen Aleena Hedgehog's children Sonic, Sonia and Manic were born, Dr. Robotnik surfaced with an army of robots and technological machines which he used to seize power over Mobius, overthrowing Queen Aleena and making the captured city of Mobotropolis his seat of power, which he renamed Robotropolis.
Following his takeover, Robotnik made the royal family outlaws. As the source of his money, Robotnik largely ignored the aristocrats and left them to play, while the poorer inhabitants were turned into slaves using roboticization. Though many citizens are still free, they live under the constant threat of roboticization. In time however, the resistance arose to wage war with the Robotnik Empire and free Mobius, with Queen Aleena's children becoming the resistance's leaders years later.