A true surprise at the time, Sonic & Knuckles gave everyone the abillity to play as Knuckles, that could fly and climb walls, besides destroying walls. Not only that and new levels, the game came in a unique cartridge that you plugged into the Megadrive and then plug on top of it other Megadrive game carts. Specific games like Sonic 3 allowed to unlock what most consider the best ever, Sonic 3 & Knuckles, the most complete 2D Sonic ever, or the ability to play as Knuckles on Sonic 2. It was later discovered that you could plug any cart on top and pressing ABC would unlock a uniqu enew 3D bonus level. Amazing and innovative tactic from Sega on this one, that proved it worth. Also, I realy love the theme song from Hidden Palace zone.