Sonic CD, simply put, is Sonic 1, but improved in almost every respect. Sonic 1 was structured with a back-and-forth structure of levels built to take advantage of its mechanics to the max, and levels which held off on them out of fear players would be overwhelmed. Sonic CD does away with this and goes all in on building ballin' labyrinths for players to puzzle their way to speed through, and it comes out as a much more consistent product as a result. There are plenty of stages focused more on traditional platforming, but unlike in Sonic 1 where these sequences are often mandatory breaks from the regular engagement, CD uses them as punishments for a lack of engagement with the mechanics, be that through ignorance or stubbornness. Even moreso than its predecessor, Sonic CD is a game about platforming via ball physics.
This is accentuated even more by the game's secondary objective, and its most iconic addition; time travel. Every Sonic game has an "emerald hunt" so to speak; some side-quest players can engage with aside from learning to time-attack or otherwise simply beating the levels. Most of these emerald hunts across the series are quite poorly designed, being an irrelevant distraction from the core gameplay which, rather than reinforcing the core engagement, asks something of the player which has so little to do with said engagement that by the time the player is finished, the game has likely undone all its work to condition it to play it as intended. Sonic CD is one of the worst offenders in this regard with how elaborate its time travel system is. Players are asked to find signs across the level to imbue them with the power to travel forward or back in time, with which they must find a means of maintaining top speed long enough throughout the stages to warp, after which they must locate a generator in the past in every level to secure the good ending (alternatively, they can copy Sonic 1's system and take 50 rings to the end of a stage to play a min-game for one of 6 collectables). The engagement of time travel itself, being the most explicitly explorative Sonic has ever been up to this point and will continue to be for years to come, is starkly at odds with the core gameplay loop. However, unlike almost every other emerald hunt to date, it does 2 worthwhile things. Well, 1.5 things depending on how you look at it...
Firstly it forces players to become skilled at using the mechanics to maintain an uninterrupted flow-state through levels. it demands a lot of players in regards to using the mechanics, even if it's not in service of the regular course-clear structure's intentions. Well, at least in theory. With there being plenty of setpieces strewn across levels where the player can accomplish time travel with little to no effort, this smart idea goes largely to waste. Secondly, this more open structure forces a lot more creativity onto the player in how they navigate levels. Sonic 2's level structure of the top route always being the fastest which diluted it so much is extremely rare here. It is saved for a particular handful of levels to make them stand out as unique among the bunch. Because of how different parts of level weave together and interact for the purposes of not just moving forward, but in all directions, navigation is nuanced and ever-changing. While perhaps on accident, this in turn very much aids the structural intentions of the core gameplay loop. Perhaps this is the reason the levels are so much more consistent than Sonic 1's.
In short, Sonic CD plays with Sonic as a walking pinball in creative and interesting ways, demanding more of the player than Sonic 1 and its alternative in the form of Sonic 2. Not only that, it beats them both at their own game.
This is accentuated even more by the game's secondary objective, and its most iconic addition; time travel. Every Sonic game has an "emerald hunt" so to speak; some side-quest players can engage with aside from learning to time-attack or otherwise simply beating the levels. Most of these emerald hunts across the series are quite poorly designed, being an irrelevant distraction from the core gameplay which, rather than reinforcing the core engagement, asks something of the player which has so little to do with said engagement that by the time the player is finished, the game has likely undone all its work to condition it to play it as intended. Sonic CD is one of the worst offenders in this regard with how elaborate its time travel system is. Players are asked to find signs across the level to imbue them with the power to travel forward or back in time, with which they must find a means of maintaining top speed long enough throughout the stages to warp, after which they must locate a generator in the past in every level to secure the good ending (alternatively, they can copy Sonic 1's system and take 50 rings to the end of a stage to play a min-game for one of 6 collectables). The engagement of time travel itself, being the most explicitly explorative Sonic has ever been up to this point and will continue to be for years to come, is starkly at odds with the core gameplay loop. However, unlike almost every other emerald hunt to date, it does 2 worthwhile things. Well, 1.5 things depending on how you look at it...
Firstly it forces players to become skilled at using the mechanics to maintain an uninterrupted flow-state through levels. it demands a lot of players in regards to using the mechanics, even if it's not in service of the regular course-clear structure's intentions. Well, at least in theory. With there being plenty of setpieces strewn across levels where the player can accomplish time travel with little to no effort, this smart idea goes largely to waste. Secondly, this more open structure forces a lot more creativity onto the player in how they navigate levels. Sonic 2's level structure of the top route always being the fastest which diluted it so much is extremely rare here. It is saved for a particular handful of levels to make them stand out as unique among the bunch. Because of how different parts of level weave together and interact for the purposes of not just moving forward, but in all directions, navigation is nuanced and ever-changing. While perhaps on accident, this in turn very much aids the structural intentions of the core gameplay loop. Perhaps this is the reason the levels are so much more consistent than Sonic 1's.
In short, Sonic CD plays with Sonic as a walking pinball in creative and interesting ways, demanding more of the player than Sonic 1 and its alternative in the form of Sonic 2. Not only that, it beats them both at their own game.