Sonic the Hedgehog platforming of the main levels. The 16-bit "Sonic the Hedgehog " consisted of a giant rotating maze, which many considered a major technical achievement. The most common special stages, however, were segments with the character running through a long tunnel to collect certain items. 3D "collect item" levels, as in "Sonic 3" and "Sonic & Knuckles", used the same perspective but had Sonic collecting all the blue-colored orbs on the surface of a giant sphere. Finally, "Sonic Advance 2" employed a 3D ring-collecting stage, and "Sonic Chaos" ("Sonic and Tails" in Japan) used a variety of gimmicks for its levels.
Sonic the Hedgehog 2 was released in 1994, two years after "Sonic 2". The "Sonic & Knuckles" game cartridge features a special "lock-on" port into 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" whereby 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,