Knuckles the Echidna in Sonic the Hedgehog 2! Sonic & Knuckles was released in 1994, two years after Sonic 2. The Sonic & Knuckles game cartridge features a special "lock-on" port at the top which the player can insert other Genesis cartridges. Attaching Sonic 2 unlocks Knuckles the Echidna in Sonic the Hedgehog 2, a variation of Sonic 2 where by the player plays as Knuckles the Echidna, a character introduced in Sonic the Hedgehog 3 and not playable until Sonic & Knuckles. Though the game is largely identical to Sonic the Hedgehog 2, Knuckles is able to glide and climb walls, allowing him access to areas previously inaccessible to Sonic or Tails. However, he cannot jump as high, making some parts of the game, such as certain boss fights, more difficult. In addition, Knuckles restarts with the amount of rings he collected at checkpoints (if he loses a life), the options are unavailable, the special stages have slightly fewer rings required to collect than befo...