Entering Green Hill Zone via the level select takes you to just that, Green Hill Zone. But this is Sonic 2 isn't it? Of course, but don't forget that Sonic 2 was in fact built off of Sonic 1 in the same fashion that Sonic 3 was built off of Sonic 2 (See: Sonic 2 menus in Sonic 3 and Sonic 3 level select pictures.) This version of Green Hill was more than likely present in early alpha builds of Sonic 2 to test collision and the engine. In this version we see that they pretty much abandoned it as all the collision is completely fucked up.
It's very probable that once they started building Sonic 2 zones like Emerald Hill, they stopped working with GHZ. This is more than likely the last major build to ever use GHZ before it was pretty much removed.
Pretty much anything that isn't level art is messed up and garbled. The only exception seems to be the Piranha badniks, who just seem to have a screwed up pallete.
Another thing to help date this zone. The monitors use their old Sonic 1 mappings. This is due to the fact that they changed some monitor mappings around due to some additions, like the Tails 1-Up mapping. Observe
 Sonic 1 |
 Sonic 2 Pre-Beta |
 Sonic 1 |
 Sonic 2 Pre-Beta |
Rings and Shield monitors have switched places between the two games. Invincibility has been replaced by speed shoes.
 Sonic 1 |
 Sonic 2 Pre-Beta |
Much of GHZ is pretty intact, including the final boss (sort of)
Defeating the garbled crapbotnik brings you to the garbled egg capsule. Jump on top..
...and the flickies and rabbits are freed, and it's off to Chemical Plant (Marble Zone in the game)