Sonic & Knuckles (World) ROM

Sonic & Knuckles is a game that was developed and published by Sega. This game features Sonic the Hedgehog and Knuckles the Echidna (ready to defeat Dr. Robotnik).

Probably the hardest though one of the best Sonic games for the Sega Genesis. Long platform stages that will take all the 10 minutes you have to complete the stage or closely...
Highly recommended for hardcore Sonic fans, in my opinion. Beginner Sonic players ought to play the first ones in the series before this one.

It's hard to talk about classic Sonic games without talking about Sonic & Knuckles. And it's not because it's the fourth installment (against what Sega might think), but because the way this game wraps everything up.
Sonic & Knuckles is the second 1994 Sega Genesis/Megadrive game to the Sonic series. It works as the direct follow-up of Sonic 3. It has the same engine, and the same gameplay (the fire, water and magnetic shields are available, and the Insta-Shield works for Sonic too), with a minor difference.
In Sonic 3, you could play as Sonic and/or Tails, and Knuckles only hindered your progress. Now, you can play as Sonic or Knuckles, and each of them have a different path, and, in some cases, different boss fights. Knuckles shines with its own light, as he's able to glide on the air freely (and at your command), and also he can grab on to some walls and climb them without major effort. He can also punch through some walls which Sonic can't get thru.
After the events of Sonic 3, Sonic falls to Mushroom Hill Zone, Knuckles finally understands that Robotnik is the bad guy and the story goes on, until the final confrontation in, once and again, the Death Egg against the evil Dr. Ivo Robotnik (Eggman in Japan).
This game is quite long by itself, comprising 7 zones total, and a showoff against Eggman. By itself, it doesn't contains any multiplayer features, nor you can save your progress.
A peculiarity of this game's cart is that it contains a special Lock-on Cartridge technology that allowed other Genesis games to be plugged in on the top of it and complement each other. When Sonic 1 is connected on top, and A+B+C is pressed on the screen that appears, "Blue Sphere" stages, like the Special Stages in Sonic 3 can be played in succession. When Sonic 2 is connected on top, you get to play Sonic 2 with Knuckles. If you pop any other game cart other than Sonic games, you'll get a special "Blue Sphere" stage depending on the cart you plug.
However, the main feature of this game is unlocked if you pop Sonic 3 on top of S&K. The full game, Sonic 3 & Knuckles, as it was envisioned by its producers, will be available for you to play.