Here are screenshots of the unfinished Hidden Palace level from Sonic 2. This is what you get when you use the Action Replay code FFFE1 00008.
There are also ways to get the unfinished Hidden Palace on the two most popular emulators, KGen and Genecyst. Click on the emulator name to get instructions for that emulator.
With the release of the Sonic 2 beta ROM, however, many of the secrets of Hidden palace were finally revealed! I won't post the ROM here for legal reasons, but by now it's been put up all over the net! I know Simon Wai's Sonic 2 Beta page carries it though.
'Oil Palace' Zone hack
Interesting Hidden Palace locations
This is what you see at the start of the level. The floors aren't implemented, meaning that you just fall straight to the bottom of the screen and die!
(unless you have debug on)
This is a comparison screenshot. On the left is a picture taken from a magazine, of what the Hidden Palace zone was originally intended to look like. On the right is a screenshot of the same location in the unfinished level. As you can see, the bridge is implemented, but none of the graphics or other floors are implemented.
This is the Emerald Hill zone, but with Wood Zone's color palette!
To get this on Genecyst or KGen, follow the instructions for the Hidden Palace hack but enter the value 02 instead of 08.
I think Sonic Team had intended to put other levels into Sonic 2, but never got round to it. However, the hangovers remain. For example, the unfinished Hidden Palace level and the color palette from the 'wierd' Emerald Hill (which I think is Wood Zone's palette). When you play the game normally, it just skips these levels.
Click here to read my theory on how these levels bit the dust.
Sonic beta pictures have now moved to the Sonic Beta page.