Since 'Sonic & Knuckles' is the first game in which the red echidna became a playable character, and since Mushroom Hill Zone is the first level of that game, the stage can easily be seen as Knuckles' debut - his Green Hill Zone, if you will. The fact that Knuckles first enters the story on the Mushroom Planet is entirely appropriate, given that the locale is clearly inspired by the 'Sonic & Knuckles' level Mushroom Hill Zone.
