Sonic & Knuckles -

Combining the two games creates the definitive experience, allowing players to play through all 14 zones as Sonic, Tails, or Knuckles, with the ability to save progress.

Inserting almost any other Genesis cartridge (including the original Sonic 1 ) grants access to thousands of unique "Blue Sphere" special stages. Gameplay and Story Sonic & Knuckles

Locking on Sonic 2 allows players to play through that entire game as Knuckles the Echidna , utilizing his unique abilities to find secrets unreachable by Sonic. Combining the two games creates the definitive experience,