Rather than a straight port or an emulated title, this version of the 1993 Sonic CD game is a completely redone version for modern platforms and based on a fanmade Retro engine. The original version was released for the SEGA CD and later also ported to Windows. This version supports a whole host of more modern platforms as well as a complete re-release for the Windows platform.
It has many changes and additions, such as true 16:9 aspect ratio (instead of stretched 4:3 like the ports of other Sonic games), the choice between both the Japanese/European and American soundtracks, a constant framerate of 60 frames per second, several bugfixes, an option to select between the original CD's Spin Dash or the Sonic 2 Spin Dash and the inclusion of Tails (with his Sonic 3 abilities) as an unlockable character.
The game adds leaderboards for the Single Player and Time Attack modes. It also regains the original game's extra modes, the D.A. Garden and Visual Mode, and also includes the Sound Test and Stage Select modes as unlockables, unlike the original in which you had to input a cheat code to enter them.