Hidden Palace Zone (Sonic the Hedgehog 2)

Hidden Palace Zone is a mysterious zone which was originally removed late in the development of Sonic the Hedgehog 2 for the Sega Mega Drive/Genesis. It is an underground cavern full of water featuring sparkling gems and rocky regal structures, possibly ruins of some ancient civilization. A similar stage with the same name later appeared officially in the game Sonic & Knuckles, serving as its inspiration. It was among the last levels deleted from the game.

Eventually, this version of Hidden Palace Zone was officially completed and included in the 2013 re-release of Sonic the Hedgehog 2 for iOS and Android devices.

Development
A picture of Hidden Palace Zone was sent to various magazines to promote Sonic the Hedgehog 2. For years, this is the most that players had seen of the level. It was determined not to be a screenshot from a prototype for several reasons: Sonic's sprite is different than any of those seen in earlier versions of Sonic the Hedgehog 2, Sonic is off-center, he is running in mid air, the use of background is different, the placement of two enemies at the bottom is off, the HUD is not italicized, etc. These are some of the same characteristics seen in other promotional mock-up pictures sent out by Sega during the same time frame.

An interview with Yuji Naka later revealed that Hidden Palace Zone was originally intended to be a level where the player would be warped to after collecting all seven Chaos Emeralds. Once there, they would receive the power to transform into Super Sonic. The idea was scrapped and the ability was given to the player immediately. In ICEknight's interview with Craig Stitt (the level artist), he mentioned that it started as two acts but then switched to a single act before being cancelled. He expressed dissatisfaction with the decision to remove the zone just a few days before the game was declared complete.

Playability
Players first noticed a somber music track listed as #10 did not appear anywhere else in the game, leading some to wonder if it was related to the elusive zone that was exposed in preview images. When Sonic the Hedgehog 3 was released, the level icon for Hidden Palace was spotted in the standalone Sound Test option (as it was directly built off the prequel's engine), suggesting that remnants of Hidden Palace still existed in the game data of Sonic the Hedgehog 2. It was eventually discovered that players can still access a garbled version of it with a cheat device, confirming that the unused song was intended for this zone in some late form. However, the zone was overwritten with a vaguely-defined Oil Ocean object list, as the player falls to their doom when the zone is accessed.

When the Simon Wai prototype was eventually leaked, a much more playable (but still obviously incomplete) version of Hidden Palace Zone was made available for the first time. Only a small part of the level was actually finished, as it cuts off after a certain point. A slot for a second act was also found at this point of development, but that had even less work done, as it was a copy of the first act with an empty object layout. Many parts of the brief level were once grounds for debate, such as the so-called "Master Emerald" which blocks a speed pipe in the prototype. In actuality, this was meant to be just a breakable object similar to the ones found in Hill Top Zone. There was much speculation about other features seen in this version of the level as well, such as the large ramp at the end that cannot that cannot be climbed up without Debug Mode, and the tubes similar to the ones in Chemical Plant Zone.

One of the more infamous objects was the Tails 1-Up monitor that appears even while playing as Sonic. Despite this, it still functions as a regular 1-Up monitor. In truth, the monitor designations were re-assigned at some point and the object subtype number for the Tails 1-Up monitor was originally the object subtype number for Sonic 1-Up monitor from the first Sonic the Hedgehog. The early Nick Arcade demo, which was an even earlier prototype of Sonic the Hedgehog 2 shown to be a heavily edited Sonic 1, places Tails 1-Up monitors in Green Hill Zone where the Shield monitor should be, proving that the Tails 1-Up monitor in Hidden Palace Zone was meant to be something else. The Tails 1-Up monitor must have been placed very early in the game's development cycle (before the programming for the object subtypes was changed), as it was one of the first levels to be worked on. Had the level originally been worked on further, it is certain the Tails 1-Up monitor would have been swapped with the proper Sonic 1-Up monitor. The Shield monitor in the beginning was also presumably meant to be a Super Ring monitor, based on the changes in object subtype numbers.

Final
Despite Hidden Palace Zone being scrapped, it still exists in the final version. It is accessible by using the Game Genie code ACLA-ATD4. From here, enter the Level Select code (19, 65, 09, 17 with C in the sound test screen). Go back to the title, and hold + Start to access the Level Select screen. Then select Death Egg Zone.

Almost all of the artwork is gone in the final game, and with this goes collision data, making the zone virtually unplayable without enabling Debug Mode (1, 9, 9, 2, 1, 1, 2, 4 with C at the level select screen). The background music is the unused #10 from the Sound Test. All the placeable items in Debug Mode are from Oil Ocean Zone. This zone has two acts; the first act has items (including the Tails monitor) and some complete and incomplete sections, while the second act is the same as the first but without any objects. It is impossible to complete the level without placing a capsule with Debug Mode. "Beating" both acts will take the player to Oil Ocean Zone.

2013 re-release
In the 2013 re-release, this zone can be accessed in Mystic Cave Zone Act 2 by falling through a pit (this is the infamous inescapable spike pit in the original, but the spikes were removed in the re-release) around halfway through the act. The zone is completely optional.

Description
The zone begins with the player character falling through the pit, similar to the intro of Lava Reef Zone in Sonic & Knuckles. The background music is the multiplayer music of Mystic Cave Zone. While pieces of the level design are borrowed from pre-release versions of the zone, it is for all intents and purposes a complete re-imagining of the level. In addition to some familiar features, there are objects not previously seen in any version of the game, such as a jellyfish-type Badnik that acts similar to Grabber, fully functional water slides, the unscalable diagonal tunnels given a propelled zipline device, spikey mines that are swung back and forth and a unique Dr. Robotnik boss fight (dubbed Brass Eggman) involving musical instruments, explosive debris and water. The Tails 1-Up monitor from the prototype is also gone and replaced with a shortcut just beyond the space it occupied and the "Master Emerald" objects now conceal springs. When the Capsule is opened and the zone is cleared, the player proceeds to Oil Ocean Zone, skipping the remainder of Mystic Cave.

Badniks
In the Simon Wai prototype, Hidden Palace Zone only has two Badniks by that point of development: Redz and BBat. Through hacking, the stegosaurus Badnik from magazine previews (Stego) was found, and both it and BFish were found in the Debug Mode of the earlier "Nick Arcade" prototype (which was leaked much later). In the 2013 re-release, the stegosaurus was reinserted and the aquatic robot was replaced by the previously unseen Jellies.

Boss
In this boss fight, Eggman flies in front of a giant archaic organ. The massive trombone horn on his Egg Mobile will activate every few seconds. He begins with four descending notes, causing four hazardous spiked balls to fall from the ceiling and land on the water below the arena. The water will then rise over the platform, taking the floating balls with them for a second or two. After being lowered to the original water level, the balls will detonate and emit a vertical splash that must be avoided. After a few more seconds, a long and loud bellow will emit from Eggman's horn, causing the screen to shake until one larger spike ball descends.

Proto Palace Zone
Despite the new layout of Hidden Palace Zone being added to the 2013 re-release of the game, a copy of the original Hidden Palace Zone prior to its deletion (under the name "Proto Palace Zone") has also been added but is not accessible via regular gameplay.

Trivia

 * This is one of the two Zones in Sonic the Hedgehog 2 not to be featured in Boss Attack Zone, the other being Sky Chase Zone (three if counting Proto Palace Zone).
 * When using Debug mode in the original Hidden Palace, it will strangely give you the sprite set for Oil Ocean Zone. The sprites for it will also appear glitched up, and when using the breakable box things, it will make you speed forward in spin dash state.  The only way to stop it is by using Debug Mode.

Music
2013 re-release= Beta=