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Sonic entering the Hidden Palace for the first time.
Hidden Palace Zone in Sonic & Knuckles is a transitional level between Lava Reef Zone and Sky Sanctuary Zone. It consists of a single act, and is the shortest level in the game. It is also the only level in any of the Mega Drive-era Sonic the Hedgehog games that is devoted to plot progression. The stage first appeared in the Sonic 2 Beta.
Sonic, Tails and Knuckles fight infront of the mural.
Hidden Palace Zone is a short underground zone located on Angel Island which has the remnants of a magnificent palace. This palace has a mural foretelling the events at Doomsday Zone of the fight between the Great Eggman Robo (Unless you don't have the emeralds) as well as a magnicifent alter for the Master Emerald where the Chaos Emeralds can be recharged to become Super Emeralds. The cave itself also has crystals growing around it.
Fanon
The Hidden Palace Zone in user:Mystic Monkey fiction
Somewhere inside Angel Island is a large cave where crystals grow. This cave is also where the ruins of an Echidna palace stay (which resembles a blend of Aztec/Roman architecture) this palace was constructed by the survivors of the Knuckles Tribe when Angel Island arose from the Earth.
The palace still has the mural as well as a secret library which holds records of history and lore of the ancient times.
The palace ruins also have a place for the Master Emerald. Knuckles his the Master Emerald in the palace when he first met Sonic and thought he was a threat.
Now the Hidden Palace is a back-up location for the floating island which carries the original Master Emerald shrine as seen in Sonic Adventure. The small island can go inside Angel Island into the Hidden Palace Zone.