Prototype Zones/Emerald Hill
From Sonic Retro
Green Hill Zone
Nick Arcade Prototype
As with Simon Wai's prototype, Emerald Hill is the most complete Zone in the ROM. It's the only one that can actually be completed, and it's the only zone with a boss. (Of course, we're not counting Green Hill Zone.)
The first thing you notice when you play this zone (by pressing start or by selecting it from the level select) is that Tails is missing. Emerald Hill is the only zone in the ROM that does this. If this is indeed the Nick Arcade prototype, Tails was likely disabled to prevent him from interfering with the 25 ring challenge.
Emerald Hill Zone Act 1: Final
Emerald Hill Zone Act 1: Nick Arcade Prototype
While there are plenty of layout differences between this prototype later builds, the entire Act still feels very familiar.
Note the similarity between prototype B's layout and the layout seen in the Nick Arcade video:
However, unlike in the later prototype, he explodes when you beat him! In Simon Wai's version his wheels simply fly off without any explosions. Note, however, that the explosion graphics are from Sonic 1. It seems that at some point between Prototype B and Prototype A, the old explosion graphics were removed without being replaced with new ones.
Some minor differences dotted around this act, mostly in the middle.
Simon Wai Prototype
Emerald Hill Zone was, quite reasonably, known as Green Hill Zone in the development stage. This zone is mostly completed. GHZ is also the only zone in this prototype that has a boss.