Sonic & Knuckles

Sonic & Knuckles is a platform game in the Sonic the Hedgehog series, developed by Sonic Team in collaboration with Sega Technical Institute, and published by Sega for the Sega Mega Drive/Sega Genesis in 1994. It is the direct sequel to Sonic the Hedgehog 3, which was released earlier that year. The game features unique "lock-on technology" which enables another cartridge to be plugged in via the socket on top.