Hidden Palace Zone is the fifth Zone in Sonic & Knuckles (and the eleventh of Sonic the Hedgehog 3 & Knuckles). As the shortest Zone in the game, it consists of a single act and functions as a transition between Lava Reef and Sky Sanctuary. It is also the only Zone in any of the Sega Mega Drive-era Sonic the Hedgehog games that is devoted to plot progression. It is based on a deleted Zone from Sonic the Hedgehog 2; although, that is considered a separate location as it would have likely been located on West Side Island (as the 2013 mobile re-release of the game reveals).