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 completely playable under a second player's control, as well as adding the option for players to utilize Tails' flying ability in levels. It also introduced the character, Knuckles the Echidna, who served as an additional antagonist with Doctor Robotnik for the game. "Sonic & Knuckles", another platform game in the "Sonic series", was released later in 1994. The game featured Knuckles as a playable character with gliding and wall climbing abilities and allowed gamers to plug in "Sonic the Hedgehog 3" to the top of the "Sonic and Knuckles" cartridge as part of the game's "lock on" functionality. This allowed