Gameplay
Players control Sonic as he speeds his way to end of each level, using his spin attack to fight against Dr. Robotnik's badniks. Along the way, Sonic can collect rings, which can protect him if is he hit by an enemy or obstacle, as well as items such as shields, invincibility and speed shoes. Sonic's gameplay remains similar to that of Sonic the Hedgehog but with the addition of the Spin Dash and the Super Peel Out, which lets him zoom into a quick speed from a standing point, either in a rolling or running position. The Super Peel Out is faster than the Spin Dash, but leaves Sonic more vulnerable. Each of the game's seven themed stages is divided into three "zones": two main levels and a boss level.
The main gameplay mechanic that sets Sonic CD apart from other Sonic games is a time travel system that enables players to move between different time periods within each level. By hitting posts labeled "past" or "future" and then maintaining a consistently high speed for several seconds, Sonic can move between past, present and future level variants. (The time travel posts are only present in the first two zones of each stage; boss zones always take place in the future.)
The game contains four different variants of each zone ("past", "present", "good future" and "bad future"), each of which features different graphics, music, and layouts of platforms, enemies and obstacles. By default, traveling to the future will take Sonic to a "bad future" version of the current level, an industrialized dystopia with scenery themed around neglect and decay, in which enemy robots exhibit signs of degradation. Therefore, players are encouraged to convert each zone's timeline to a "good future": a utopic technogaian scenario in which technology and nature are symbiotically fused into a sustainable, colourful environment, and in which there are no enemy robots. In each non-boss zone, a good future can be attained by traveling to the past and destroying a hidden "robot generator". If a good future is achieved in both of a stage's non-boss zones, that level's boss fight will also take place in the good future.
Similar to Sonic the Hedgehog, if a player has more than 50 rings by the end of the act, a giant ring appears that can take Sonic to a Special Stage if he jumps into it. On a three-dimensional plane, the player has a short amount of time to destroy several purple UFOs floating around the level. Time is rapidly reduced if the player runs through water, though a blue UFO which appears when time is running out can grant extra time if destroyed. If the player is able to destroy all the purple UFOs before the time runs out, a Time Stone is earned. A good ending can be achieved by collecting all seven Time Stones, or by achieving a "good future" in every act. The player will also automatically achieve a "good future" in any level if all the Time Stones have been collected.
Sonic CD contains a "backup save", using the internal Mega-CD memory or a backup RAM cartridge. The game saves after the end of each third zone (after which, a new level begins) and records the best times of the player in the time attack mode. In the 2011 remake, the game is saved at the end of each zone. The game also features an instant game over scenario: if the game is not paused and is left alone for three minutes, Sonic will leap off the screen and shout the catchphrase "I'm outta here!", and the game will instantly end, regardless of the player's number of lives.
For the 2011 remake, players are also able to control Miles "Tails" Prower after clearing the game once. He basically has his moveset from Sonic the Hedgehog 3. Just like Sonic, he can use the spin attack, spin dash, along with his ability to fly by spinning his tails like helicopter rotors and also swim, but is unable to use the Super Peel Out and cannot be used to earn achievements. Players also have the option to utilize the Spin Dash physics from Sonic 2.
Read more about this topic: Sonic The Hedgehog CD