First up let me say this - you’ve done a bloody FANTASTIC job translating the environments, mechanics and overall experience from 3D to 2D. Absolutely amazing work! The work you’ve put into the game really shows and shines on through as a pure retro sonic experience (It honestly feels like I’m playing what I wanted Sonic 4 to be in my sega megadrive days!)
With that said, I do have a few points of feedback in terms of improvements and/or critique that, in my experience, lets the game down a little.
Boss difficulty curve seems a little erratic. It seems that I had quite a bit of trouble on Green Grove act 2 boss more than I did with the following act 2 bosses, simply because of a few things, notably the horizontal speed of the ball (this was eventually learned, after a game over, but it feels way too difficult for new players and more importantly the first main boss in the game) and robotnik’s i-frames catching the player out of Sonic’s natural “bounce” on bosses - I feel this shouldn’t punish the player in terms of loss of rings/death, but something less punishing like have Sonic bounce away similar to future bosses as the current method takes away something players do naturally.
Foreground/background assets and hazards - sometimes it’s almost impossible to see a set of spikes for example. Diamond Dust act 1 had various places where the spikes would be completely camouflaged in front of the background tiles, making this purely unfair - not difficult.
The Special stages - Yes, this has been brought up a lot (and for good reason). I understand your vision that they’re supposed to be difficult and that the reward is something overpowered that practically “breaks” the game but the bonus stages are teetering more on being unfair than difficult - it’s easy when you know the patterns and have designed them but when you need extremely quick reaction times coupled with the ‘slippy’ controls in them stages, it makes for an infuriating experience. A couple suggestions that may lighten up the experience without removing the “difficulty” could be tightening up the momentum a little or slowing the rings down slightly (but adding more to the quota/having less rings appear).
Quick boss attacks - Things like the Diamond Dust act 1 boss. I notice this has been brought up a fair bit too, and as I understand the pattern and get that the idea is to avoid the lasers as they lock on to your position, the execution requires very fast reactions whilst also trying to figure out when it’s going to shoot - a recipe that is yet again, infuriating. A simple tell when the boss is about to shoot would work wonders (just a sprite flash followed by the shoot or something) and gives the player a fair warning when to dodge that incoming swift laser.
Again, I absolutely appreciate the effort you’ve put in this game, it’s probably one of the best fan games I’ve ever played. The passion and love you’ve obviously put into it is crystal clear and without a shadow of a doubt I’d recommend this to any sonic fan! If it weren’t for a few small bits that I mentioned above this would be a solid 10/10 for me!
Awesome work.