Sonic CD~!

wikipedia wrote:The main innovation of this chapter in the Sonic series is the manner in which the player can travel to three different versions of each zone, each a different time period of the same location. This is accomplished by speed posts scattered around the level, bearing the labels "Past", and "Future". After running through one of these posts, the player has to run at top speed for a few seconds without stopping, to travel into the respective time period. Because these teleports are relative, there are no "Past" signs in the Past, and no "Future" signs in the Future; that is, warping to the past in the future returns the player to the "present" time and vice versa. Each stage has three "Acts" (Although they are called "zones" in this game, see below), the third of which always takes place in the future.

The different time zones have slightly different layouts and sprite placements, as well as significant changes in the level music, art and palette. In addition, the robots within a level fall into a state of disrepair as time passes; in the present, some machines have become worn down while in the future all of them have. This affects the speed and attacking ability of the robots; some of them become completely ineffective, while others do not significantly change.

The appearance of the future changes depending on the actions of the player in the past. Hidden within the past of every level, there is a robot generating machine. If this is destroyed within a zone or all seven time stones are already collected, all of Dr. Robotnik's robots will be destroyed in the past. Should the player warp into the future, it is a "Good Future" in which there are no enemies and fewer hazards. If the machine is not destroyed, the warp will lead the player into the "Bad Future" in which Dr. Robotnik's robots run rampant, there are more hazards (though due to wear on some of the enemies, not always as many as in the past), and heavy pollution has harmed the level.
Sonic CD title screen

In addition to the robot generating machine, hidden within the past of each level is a machine which projects a hologram of Metal Sonic squashing one of that particular zone's animals underfoot. Destroying this machine causes animals to appear in the past and present levels. However the animals are always present in the Good Future, regardless of whether or not this machine was destroyed.

The third zone always takes place in the future and is mainly a short run up to the boss. Most boss battles are more elaborate than those in the other Sonic games, and typically require fewer hits than the usual 6 or 8. These boss battles, however, require more effort to actually hit Robotnik; one battle takes place on a makeshift pinball table and requires the player to use flippers to get up to Robotnik. Two battles do not involve hitting Robotnik to damage him; one takes place on a giant treadmill where the objective is to wear out Robotnik's machine by running on it, and the other is a race against Metal Sonic. The appearance of the third zone depends on the player's actions in the other two; if the player has achieved a Good Future in the other two zones (or all the time stones are collected), this zone will be a Good Future as well. However, if only one or neither stage has been made into a Good Future, the third zone will be a Bad Future. If all the third zones have Good Futures, the player is able to see the good ending but if you get all gems and all good futures you see the bonus ending.

As in Sonic the Hedgehog, special stages can be accessed at the end of each zone if the player has collected, and is holding on to at least 50 rings, whereas in the Sonic the Hedgehog 2 checkpoints are used to enter special stages. It is because of this that there are speculations that Sonic CD began development before Sonic 2. A giant ring will float above the finishing sign which Sonic can jump through to enter the special stage. They consist of a three-dimensional, flat surface. To complete a stage and collect the Time Stone reward, the player must seek and destroy six purple UFOs flying around the stage. If a UFO is destroyed, it gives a prize of either a super ring (have gold markings and give progressively larger bonuses starting with 20 rings when destroyed in series) or speed sneakers (have grey markings and temporarily boost speed). Collecting the seven time stones, only possible in the special stage, automatically guarantees that the player will reach the good ending even if one of the previously completed zones did not have a Good Future, and that all futures of upcoming zones will be good as well.The main innovation of this chapter in the Sonic series is the manner in which the player can travel to three different versions of each zone, each a different time period of the same location. This is accomplished by speed posts scattered around the level, bearing the labels "Past", and "Future". After running through one of these posts, the player has to run at top speed for a few seconds without stopping, to travel into the respective time period. Because these teleports are relative, there are no "Past" signs in the Past, and no "Future" signs in the Future; that is, warping to the past in the future returns the player to the "present" time and vice versa. Each stage has three "Acts" (Although they are called "zones" in this game, see below), the third of which always takes place in the future. The different time zones have slightly different layouts and sprite placements, as well as significant changes in the level music, art and palette. In addition, the robots within a level fall into a state of disrepair as time passes; in the present, some machines have become worn down while in the future all of them have. This affects the speed and attacking ability of the robots; some of them become completely ineffective, while others do not significantly change. The appearance of the future changes depending on the actions of the player in the past. Hidden within the past of every level, there is a robot generating machine. If this is destroyed within a zone or all seven time stones are already collected, all of Dr. Robotnik's robots will be destroyed in the past. Should the player warp into the future, it is a "Good Future" in which there are no enemies and fewer hazards. If the machine is not destroyed, the warp will lead the player into the "Bad Future" in which Dr. Robotnik's robots run rampant, there are more hazards (though due to wear on some of the enemies, not always as many as in the past), and heavy pollution has harmed the level. Sonic CD title screen In addition to the robot generating machine, hidden within the past of each level is a machine which projects a hologram of Metal Sonic squashing one of that particular zone's animals underfoot. Destroying this machine causes animals to appear in the past and present levels. However the animals are always present in the Good Future, regardless of whether or not this machine was destroyed. The third zone always takes place in the future and is mainly a short run up to the boss. Most boss battles are more elaborate than those in the other Sonic games, and typically require fewer hits than the usual 6 or 8. These boss battles, however, require more effort to actually hit Robotnik; one battle takes place on a makeshift pinball table and requires the player to use flippers to get up to Robotnik. Two battles do not involve hitting Robotnik to damage him; one takes place on a giant treadmill where the objective is to wear out Robotnik's machine by running on it, and the other is a race against Metal Sonic. The appearance of the third zone depends on the player's actions in the other two; if the player has achieved a Good Future in the other two zones (or all the time stones are collected), this zone will be a Good Future as well. However, if only one or neither stage has been made into a Good Future, the third zone will be a Bad Future. If all the third zones have Good Futures, the player is able to see the good ending but if you get all gems and all good futures you see the bonus ending. As in Sonic the Hedgehog, special stages can be accessed at the end of each zone if the player has collected, and is holding on to at least 50 rings, whereas in the Sonic the Hedgehog 2 checkpoints are used to enter special stages. It is because of this that there are speculations that Sonic CD began development before Sonic 2. A giant ring will float above the finishing sign which Sonic can jump through to enter the special stage. They consist of a three-dimensional, flat surface. To complete a stage and collect the Time Stone reward, the player must seek and destroy six purple UFOs flying around the stage. If a UFO is destroyed, it gives a prize of either a super ring (have gold markings and give progressively larger bonuses starting with 20 rings when destroyed in series) or speed sneakers (have grey markings and temporarily boost speed). Collecting the seven time stones, only possible in the special stage, automatically guarantees that the player will reach the good ending even if one of the previously completed zones did not have a Good Future, and that all
futures of upcoming zones will be good as well.