Sonic the Hedgehog is a platform game where players control the titular character Sonic, who is tasked with defeating Doctor Robotnik[e] and obtaining the six Chaos Emeralds. The game takes place from a side-scrolling perspective. He is capable of running at high speed through levels that include springs, slopes, bottomless pits and vertical loops.[5] The levels are populated with robot enemies, inside which Dr. Eggman has trapped animals; destroying a robot frees the animal, but is not necessary to complete the game.[6] His primary form of offense is a spin attack, in which he curls into a ball and spins his body, damaging enemies and certain obstacles upon collision. This may be performed by jumping or rolling.[7]
Scattered around each level are gold rings that can grant Sonic protection from a single enemy or hazard as well as an extra life if 100 are collected.[7] Also scattered throughout the level are canisters containing power-ups such as additional rings and temporary invincibility.[5] Signposts act as checkpoints where Sonic can respawn after losing a life.[8]
The game is split into six principal zones, followed by a short Final Zone. Each main zone has its own visual style, and while some enemies appear throughout, each zone has unique enemies and obstacles.[9] Each main zone is split into three acts,[5][10] all of which must be completed. At the end of each main zone's third act, the player confronts Dr. Eggman for a boss fight. For most of the fights, Eggman's vehicle is fitted with different weapons.[11] After completing the sixth zone, the player continues to the single-level Final Zone for a last encounter with Eggman inside a large machine environment.[12] A brief animation shows Sonic's return to the first zone, with animals liberated from Dr. Eggman.[citation needed]
Reaching the end of any zone's Act 1 or Act 2 with 50 rings will cause a large ring to appear through which Sonic can jump to enter a bonus stage. In them, Sonic is perpetually curled up in his Spin Attack animation, and bounces off the bumpers and walls of a fully rotating maze. In these levels, the player earns a number of continues for each multiple of 50 rings collected, but the main goal is to obtain the Chaos Emerald hidden within the maze. Colliding with any of the blocks marked "GOAL" ends the level