Here is a hack to get to the Hidden Palace Zone using Genecyst. Step-by-step instructions are given below. You will need Genecyst, the Sonic 2 ROM and a hex editor.
(This is rather complicated, so follow each instruction carefully)
1. Load Genecyst and the Sonic 2 ROM
2. Press 0 on the title screen. (This will select Save slot 0)
3. Do the Level select and Debug codes as normal.
4. On the level select, hold A and START on Emerald Hill 1.
5. As soon as the zone title disappears, press F5 to save the
snap file.
Now comes the tricky part.
6. Go into DOS, and use a hex editor to load
thefile SONIC2.GS0.
7. Change the byte at address 0012280, offset 8to
08.
8. Save your changes and exit the hex editor.
9. Restart Genecyst, load the Sonic 2 ROM and press F7 to load
your game.
(You will notice that some of the scenery will have gone weird)
10. Allow Sonic to die.
When the level restarts, you will have the unfinished HIDDEN PALACE ZONE ACT 1!