The unfinished Hidden Palace Zone in Sonic 2 is a cavern filled with jewels. It has been compared to Sonic the Hedgehog 3's Ice Cap Zone due to graphical similarities. Hidden Palace took its name from the underground cavern near Mt. Arakai in central Japan. It too is filled with precious stones, as well as ancient rock formations.[citation needed]
In the beta version of Sonic 2 (which is publicly available and playable on an emulator as it was stolen from a game show in 1992), Hidden Palace is an incomplete level. Act 2 starts with Sonic stuck inside a wall, and using the debug mode to move Sonic outside the wall reveals that the act is identical to Act 1, but without any objects. Act 1 cannot be completed at all, as it has no end level signpost or capsule. Using debug mode allows the player to get into a second part of the level, due to a steep slope that cannot be climbed up, and even when climbed up reaches to the top of the level map and leads no further. Sonic Research Zone has found that if you used debug to place a platform on this slope, Sonic can walk up the slope as if it were flat ground.
The level was one of the earliest shown to the public during production of the game. It appeared alongside the famous Desert Zone in two mock-up images that were some of the first, if not the first, shown for the game. The area in the mock-up is found in the beta, and via ROM hacking, the rhino and tricerotops badniks were also found to still be present in the game data. The game was continually used in promotion for some time. Time issues have been stated for the reason for cutting the level as has fun factor.
There are three areas that cannot be reached without Debug: a small underwater area that leads to an item box shortly after you leave the tube is under a patch of green floor (which is different to the rest and looks similar to the breakable areas of floor in the Marble Zone of Sonic the Hedgehog), a tube that leads to death below the large emerald, and an area past the slope that cannot be ran up. The latter is assumed to be the top of that slope. It has water slides on it like some areas of the Labyrinth Zone, but Sonic does not slide down the water. At one end is the top of the slope, and at the other just water and an empty space that leads to the edge of the map. A fourth area is in fact accessible without debug, though it is just very hard, requiring you to jump to a platform directly below the one you are standing on. It has been suggested that a bridge would have been put there.
The level features a large emerald above a tube, which has caused much confusion and speculation, as many have confused it with the Master Emerald, the large emerald guarded by Knuckles the Echidna that keeps the Angel Island afloat, leading to various speculations including Knuckles being planned for Sonic 2 and Tails being the guardian of the Master Emerald in Sonic 2. The level designer, however, revealed that it was simply just a breakable object, similar to the rocks in the Hill Top Zone or Angel Island Zone, only with a purpose.
There is one extra life item box in the level, and it is a Tails icon box, leading some to speculate that the level was a Tails Only level. However, suggestions have been made that the level was also connected to Super Sonic; one by a member of the Sonic 2 team.
Pieces of HPZ are left in the finished Sonic 2 game, including the music (Sound test #10), the sprite layout and code, the title card, the palette, the level select icon (which also appears in Sonic the Hedgehog 3's level select along with all the other Sonic 2 level select icons), and parts of the level data (including size, bounds, and water). Using a Game Genie or mod through emulation, one can enter the zone in the final version. However, the art itself has been removed, and so this results in a glitchy-looking (though still playable) screen causing Sonic to immediately fall to his death.
When the Beta was made public, the appearance of Hidden Palace caused an uproar in the hacking community, which eventually resulted in it being restored to full playability in the fanmade hack, Sonic 2: Long Version, complete with an Act 2 boss that behaves in a way similar to the one in Mystic Cave Zone.
