Hidden Palace Zone (Sonic the Hedgehog 2)

Hidden Palace Zone is the most famous of the "lost levels" from Sonic 2 Simon Wai prototype, partly because it remains in a corrupted form on the final version of the cart, but also because of the number of magazines it was shown in. It is an underground level with water, featuring sparkling gems. There is a glowing bridge that is actually the same object as the bridge in Emerald Hill Zone but it checks the level number to make it glow. The idea of a bridge that will glow when Sonic runs on it was reused in Sonic 3's IceCap Zone. There is also a large ramp in the level that can't be climbed up (However, if a platform is placed on the ramp with debug mode it can be walked up normally). A second act can be accessed also, but it is nearly empty.

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 the 7 Chaos Emeralds. Once there, they would receive the power to morph into Super Sonic. The idea was ultimately killed and the ability was given to the player immediately. In ICEknight's interview with Craig Stitt he mentioned that it started as two acts but then switched to a single act before being cancelled.

Many parts of the level were once grounds for debate, such as the so called "Master Emerald" which blocks a speed pipe. In actuality, this was meant to be just a breakable object similar to the ones found in Hill Top Zone. There were also theories about the level being tied to Tails somehow, due to the Tails 1-up monitor in this level. There have been other theories about the level but most theories have been generally accepted as false.

Some hacks, like Sonic 2 Delta, have attempted to restore the level using the graphics from the prototype version and level map and object data from the graphic-less version in the final release.

Magazine Preview
A picture of Hidden Palace Zone, shown at left, was sent to various magazines to promote Sonic the Hedgehog 2. 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 a Sonic 2 prototype, Sonic is off-centered, he's running in mid air, the use of a different background, the placement of two enemies at the bottom, the non-italicized HUD, etc. These are some of the same characteristics seen in other promotional mock-up pictures sent out by Sega during the same timeframe.

Misplaced Monitors
There was much speculation about the Tails 1-up monitor until the truth behind it was uncovered. The monitor designations were reassigned and the object subtype number for the Tails 1-up monitor was originally the object subtype number for Sonic 1-up monitor in Sonic 1. The early Nick Arcade prototype (which was shown to be an edited Sonic 1) places Tails 1-up monitors in Green Hill Zone where the shield monitor should be. This proves that the Tails 1-up monitor in Hidden Palace Zone was meant to be a shield monitor instead of a Tails monitor. The Tails 1-up monitor must have been placed early in the games development cycle (before the programming for the object subtypes was changed) as it was one of the first levels to be worked on as the Nick Arcade prototype shows. Had the level been worked on further it's 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 10 rings monitor, based on the changes in object subtype numbers.

In the Final Release of Sonic the Hedgehog 2
Hidden Palace is also playable in the final version of Sonic 2 if the Game Genie code ACLA-ATD4 is entered. From there enter the Level Select code (19, 65, 09, 17 in that order on Sound Test in Options) then go to the Level Select screen (+ at Main Menu while cursor is at "1 Player") and select Death Egg Zone. The level mainly is a green glitch with water at the bottom, and below the water is a bottomless pit. Interestingly the unused music track 10 from the sound test is played here. In previous pre-releases of Sonic 2 another track is played. There is no end sign in the level, so the only way to get to Act 2 is if an animal capsule is placed using the Debug Mode (1, 9, 9, 2, 1, 1, 2, 4 in that order at the Level Select Screen Sound Test and hold +  on the highlighted level) and jumped on. When both acts are completed, the game then loads Oil Ocean Zone.File:HPZ1inSonic2Final-1.png|The art for the title card is intact, but the level art is completely gone