There is now a secret final boss in the game, the boss is Knuckles from Sonic 3.
Put in the Sound Test: 1, 8, 2, 3, 1, 7 and enter the Level Select screen. Select Death Egg Zone, make sure you have the Chaos Emeralds.
Enter the Death Egg Zone as you beat the Mecha Sonic boss normally. Then, chase Eggman while jumping. Then, you now see Robotnik running out of the stage with no Death Egg Robot and just Knuckles waiting for you.
Tails follows you in Death Egg Zone.
Alex Kidd is playable by hacking, but is the hardest character.
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