Sonic the Hedgehog CD is held in high regard by Sonic fans. The game plays out in standard Sonic fashion, but the main feature to set this apart from other Sonic games is the concept of time-travel.
As usual, the zones contain two standard acts then concludes with a boss stage. The zones contain many enemies, rings and secrets, and the level designs themselves are straight-forward and easy. The bosses are varied and aren't always the traditional battles with Dr Robotnik. One has you navigating a pinball-style arena, another sees you chasing him around a cavernous area, another has you running along a conveyor belt to generate friction whilst dodging bombs, and there's a fairly challenging race against Metal Sonic.
There are essentially four time periods; present, past, good future and bad future. You always start in the Present in the first two acts, and the Future on the third act (boss battle). Travelling through time is a two step process. Firstly, you have to pass a time post which are labelled either 'Past' or 'Future' respectively. Secondly, you must achieve high speed for a small amount of time. Once you have done this, the stage layout will change as well as the aesthetics and enemies.
There are two ways of creating a 'good future'. The first one involves finding and destroying Robotnik's machine in the past. An alternate way is to collect the seven time stones. This is like finding the Chaos Emerald in previous games where you are required to finish a level with 50 rings and jump into the giant ring at the end of the level. This transports Sonic to the special stage where he needs to run around a 3D environment and destroy all the UFOs within a time limit. It's easier said than done though; because it's easy to misjudge the location of the UFOs in the 3D plane and falling into the water will decrease your time limit. There are plenty of hazards to make your task difficult too like bumpers and boost pads which often send you towards the water.
Sonic can use his Spin Dash and the Super Peel Out to gain instant speed. Spin Dash gives Sonic speed in a rolling position, whereas the Super Peel Out keeps him standing. The Super Peel Out is faster than the Spin Dash, but leaves Sonic more vulnerable.
Initially, you can only play as Sonic, but Tails is unlockable after completing the game which is an addition made for the modern versions of the game. Tails' has his flight ability which can help exploration in order to locate the machines or collect more rings.
There are 3 graphical filters to apply; Sharp, Smooth, Nostalgia. Therefore, you can play the game in a more faithful style, or go for a sharper, modern approach.
Due to the time-travel mechanic, the game naturally has more replay value and the addition of Tails increases this further. The actual game lasts just over an hour which is pretty standard for Sonic games. The easy difficulty makes it less frustrating to play, but it doesn't really give you much of a challenge. The fact that it saves after every level means that a challenge wouldn't have been too frustrating, unlike the old Sonic games where losing all your lives sends you back to the start. There's a lack of bottomless pits in the levels, so there's less chance of instant death. It feels a lot like Sonic Chaos where you can speed through the levels but also have the option to explore (which is required if you want to complete the game in its intended way).
I normally find that Sonic games suffer from a harsh difficulty curve; starting out easy but become infuriating in the final part of the game. In Sonic CD, it's easy all the way through which will disappoint those seeking a challenge. It does contain all the standard aspects that make a great Sonic game, and it does remind me of Sonic Chaos minus the difficult boss fights. As it goes, Sonic CD's bosses are rather easy, even the last boss which makes for a disappointing climax.