up version of Super Sonic & Knuckles. If you plugged in Sonic  2, you could play Sonic 2 with Knuckles, in which they added extra  areas that only Knuckles could reach, and it made the last boos more  difficult as Knuckles cant jump as high as Sonic & Tails which  limited the opportunities to attack. If you plugged in Sonic 1, you  can play 1000 different 3D special stages after holding A, B & C and  pressing START. Plugging in any other game would allow you to play 1  3D special stage, but that same stage is available when locking on  Sonic 1.