Hidden Palace Zone

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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.

Prophacy

Sonic, Tails and Knuckles fight infront of the mural.

One of the distinct features of this zone is the mural foretelling the events at Doomsday Zone of the fight between the Great Eggman Robo (Unless you don't have the emeralds).

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.