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.
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.
It appears as a level in the game. It is the second Act of Angel Island. Hidden Palace takes place somewhere below the surface of the Angel Island, inside a temple of ruins. There are several murals and bridges on the temples (pictured on the right). Several Gravitus and Eggman Nega's robots appear in this level.
An accessable location inside Angel Island which will serve as base for Sonic and his friends. It is a large cave where crystals grow. This cave is also where the ruins of an Echidna palace stay (with an architectural blend of ancient greek and aztec). 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 hid the Master Emerald in the palace when he first met Sonic and thought he was a threat. It is during Ultimate Sonic where Knuckles hide the Master Emerald when it looses it's power, being the back-up location for the small floating island which carries the Emerald altar (The small island holding the Emerald Shrine as seen in Sonic Adventure, could be accessed by a wooden bridge).
Action Stages are: