Sonic the Hedgehog is a 2D side-scrolling platform game. The gameplay centers on Sonic the Hedgehog's ability to run at high speed through levels that include springs, slopes, bottomless pits, and vertical loops.[2] The levels are populated with hazards in the form of robots inside which Dr. Robotnik has trapped animals. Destroying a robot frees the creature, but is not necessary to complete the game.[3] The player must also avoid touching spikes, falling into bottomless pits, and being crushed by moving walls or platforms, as well as drowning, which may be prevented by breathing air bubbles from vents.[4] Sonic's main means of attack is the Spin Attack, in which he curls into a ball and rapidly spins his body, damaging enemies and certain obstacles upon collision. This may be performed by jumping or by rolling on the ground.[5]
At the start of the game, the player is given three lives, each of which may be lost if Sonic collides with hazardous enemies or objects while in possession of no rings, falls to the bottom of the level screen, or exceeds an act's ten-minute time limit. Signposts act as checkpoints to allow Sonic to return to the most recently activated post when he loses a life.[6] The time resets when he returns to the checkpoint.[7] The game ends when the player runs out of lives,[8] although the player may return to the beginning of the act with three lives if the player has any continues.[7]
Scattered around each level are gold rings.[5] Collecting 100 rings rewards the player with an extra life.[5] Rings act as a layer of protection against hazards: if Sonic holds at least one ring when he collides with an enemy or dangerous obstacle, all his rings will scatter.[3] He can recollect some of them again before they disappear. If he is hit without holding any rings, he loses a life.[5] Shields and temporary invincibility can be collected to provide additional layers of protection, but certain hazards, such as drowning, being crushed, falling into bottomless pits, or running out of time, kill Sonic regardless of rings or other protection.[2]
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,[2][4] all of which must be completed. At the end of each main zone's third act, the player confronts Dr. Robotnik for a boss fight. For most of the fights, Robotnik's vehicle is fitted with different weapons.[10]After completing the sixth zone, the player continues directly to the single-level "Final Zone" for a last encounter with Robotnik inside a large machine environment.[7] Destroying Robotnik's machine ends the game. A brief animation shows Sonic's return to the first zone, with animals liberated from Dr. Robotnik.
Optionally, if Sonic reaches the end of any zone's Act 1 or Act 2 while holding at least 50 rings, a large ring appears through which he can jump to enter a "special stage". In the Special Stages, Sonic is continually 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.
