Sonic the Hedgehog is a 1991 2D platformer that kicked off the Sonic franchise and boost Sega higher into the gaming market. The object of the game is to stop Dr. Robotnik from taking over South Island.
Dr. Robotnik, a mad scientist, has uncovered the six Chaos Emeralds and plans to use their energy to rule South Island. To further carry out this plan, he builds a giant metallic base right on South Island, captures the animal residents of the Island and turns them into his robotic slaves. A blue hedgehog named Sonic finds that his animal friends have disappeared, and later discovers what Robotnik is up to when he finds that his friends have become robots and have turned against him. Sonic races through the six Zones of South Island to collect the Chaos Emeralds, stop Robotnik and free the residents of the Island.
The gameplay of Sonic the Hedgehog is intended to be simple and fast-paced; run through the levels while collecting rings and overcoming robots and obstacles. Each Zone is divided into three Acts, with every third Act having a boss encounter with Robotnik. Sonic has the ability to curl into a ball, and Sonic can use this to destroy robots be jumping on them or rolling into them. Scattered throughout levels are springboards, loop-de-loops and monitors that serve as power-ups. One of the main obstacles in the game is spikes, which are often found in groups of three. Rings act as health for Sonic; you need at least one ring to take damage without dying. If Sonic comes in contact with spikes or other obstacles or touches an enemy while not in ball form, all of his rings will scatter across the ground, giving Sonic the advantage of being able to collect some Rings back. If Sonic takes damage when he has no Rings at all, he will lose a life. Lampposts can also be found within levels, acting as Checkpoints; If Sonic loses a life, he will not start back at the beginning of the Act, but will instead start back at the last Lamppost he crossed. At the end of every first and second Act of a Zone sits a Goal Plate, which marks the end of the Act when Sonic runs past it; hidden Bonus Points can also be found past the Goal Plate - 100 points, 1000 points or 10000 points.
- Green Hill Zone: A tropical bayside with green grass and the famous chequered soil. Though this Zone is abundant in spikes, it is a fairly easy level, being as it is the first Zone in the game.
- Marble Zone: An ancient ruin on the outside, and a dark dungeon on the inside. Lava is the main element in this Zone, which sometimes have fireballs that jump out and back into the lava.
- Spring Yard Zone: A bizarre pinball arena seemingly set in an urban park. Sonic can become like a pinball when he bounces on the bumpers scattered throughout the Zone, and Springs are also common here.
- Labyrinth Zone: An ancient, maze-like, underground ruin halfway flooded with water. Sonic can only stay underwater for 30 seconds before he drowns, and a countdown will start after 18 seconds have passed. Sonic can breathe in air bubbles found underwater to keep from drowning.
- Star Light Zone: A peaceful city highway with many loop-de-loops and high-speed sections. One of the prominent obstacles in this level are seesaws that hold spikeballs, which Sonic can use to propel himself to higher ground.
- Scrap Brain Zone: Dr. Robotnik's metallic factory and base of operations. There are many dangerous traps in this level that require percision to overcome, and just about every robotic enemy that was in any previous Zone now appears here. Act 1 of this Zone takes place outside the base, showing acres of buildings and power plants. Act 2 takes place inside the factory which appears to be a vast laboratory. Act 3 appears to resemble Labyrinth Zone, but with a different color palette.
- Final Zone: Essentially the boss fight for Scrap Brain Zone, this one-act Zone sees Sonic engaging in battle with Robotnik for the last time. This level is a room with four crushing pistons; two on the floor and two on the ceiling. Robotnik's attack strategy is to send down two pistons at one time, with one of the pistons having Robotnik inside. Sonic must either roll or jump into the piston that is holding Robotnik. Between each of these attacks by Robotnik, he will generate four electric spheres that home in on Sonic, who must avoid them. There are no Rings in this level, making the fight more difficult.
If Sonic manages to collect 50 or more Rings in every first or second Act of a Zone (with the exception of Scrap Brain Zone), a Giant Ring will appear past the Goal Plate, which Sonic can jump into to access one of six Special Stages. Here, Sonic navigates through a rotating maze where he can collect Rings, Continues and, most importantly, a Chaos Emerald at the end of the maze. To add challenge to these levels, there are Goal Pits that send Sonic out of the Special Stage with no Chaos Emerald if he falls in. Special Stages play an important part in this game, as they are required to collect the Chaos Emeralds and achieve the game's Good Ending.