Sonic & Knuckles is a 1994 platform video game developed and published by Sega. The sequel to Sonic the Hedgehog 3 (1994), players control Sonic the Hedgehog and Knuckles the Echidna in their quests to save Angel Island; Sonic tries to prevent Doctor Robotnik (Dr. Eggman) from relaunching his orbital weapon, the Death Egg, while Knuckles scuffles with Robotnik's minion, EggRobo. Like previous Sonic games, players traverse side-scrolling levels at high speeds while collecting rings and defeating enemies. Sega Technical Institute developed Sonic & Knuckles simultaneously alongside Sonic 3; they were planned as a single game until time constraints and cartridge costs forced the developers to split it. The Sonic & Knuckles cartridge features an adapter that allows players to connect the Sonic the Hedgehog 3 cartridge, creating a combined game, Sonic 3 & Knuckles. Sonic the Hedgehog 2 (1992) can also be attached, allowing players to control Knuckles in Sonic 2 stages. Sonic & Knuckles was released for the Sega Genesis on October 18, 1994. It sold 1.24 million copies and received positive reviews; critics were impressed with the replay value and lock-on technology, despite its similarity to Sonic 3. Alongside Sonic 3, a Windows port was released through the Sonic & Knuckles Collection (1997). It has also been rereleased through various compilations and on digital platforms such as Xbox Live Arcade and the Wii's Virtual Console, while it was bundled with Sonic 3 on Steam.