Sonic CD, the entirely forgotten Sonic title. When Sega decided to add on the new CD drive hardware add-on for the Sega Genesis back in 1993, the idea of playing ANYTHING from a CD was entirely new. The CD drive add-on to the Genesis was a complete failure in its day and all of the titles that used it were instantaneously forgotten. Sega's mascot, Sonic the Hedgehog, was one of the many of them with Sonic CD.
If you've played Sonic the Hedgehog back in its two-dimensional days, then Sonic CD shouldn't look or feel any different than the other 90s entries of the series. There was one major difference to this besides being played on a CD instead of a cartridge, and it was a new mechanic implemented into the game. In every level, there's a gateway that players can pass through that will allow you to travel backward or forward in time. When I first encountered it, I was entirely confused and had no idea what exactly it was and why my screen transitioned into another realm when I passed through the gate at high speeds. If you slowly walk through the gateway, nothing will happen and you will remain in the "present." When you are in certain aspects of time, the level of which you are playing in will completely change. You'll encounter different enemies, different music, and different landscapes. Basically, there are three levels in one for every act you play through, and if you play the game in a specific fashion, the ending of the game can be completely altered where things that could have happened never did or vice versa. Sonic the Timecop.
Even today, just the aspect of such a thing being accomplished in a platforming title is truly breathtaking. Seeing a game completely change the perspective of what you were just playing is a really nifty idea. I'm not really sure if a time mechanic that was present in Sonic CD has ever been replicated or upstaged whatsoever. If not, someone get on that! Sonic CD just isn't a direct port either. Sonic CD is also HD. The game is immediately tailored with three video filters (Sharp, Smooth, and Nostalgic). My personal preference is obviously the much pixelated Nostalgia filter because...I'm nostalgic...DUH! If you want to see the game in its high definition glory, Sharp is the setting you'd like to pick. The game does look VERY nice in HD with it's vibrant color pallette. Oh, and widescreen functionality has also been implemented. Another cool thing that I like is how you can choose between the Japanese and American soundtrack for the game. I never thought a soundtrack would add replay value to a video game. I'll say my personal preference again. I prefer the American soundtrack to the game, not because I'm American, but because the soundtrack was much more funky in the American version than the Japanese version. Although some Japanese tracks for certain levels were better than the American tracks. The controls are very intuitive as they always have been in the 90s Sonic titles so there's not much else to go on there.
What else was brought into Sonic CD's HD foray was twelve Achievements, leaderboards, and a Time Attack mode. Oh, and Tails. Tails was originally not an addition into the Sega CD edition of Sonic CD. Tails is unlocked once you beat the game with Sonic.