I absolutely love the physics in this rendition of Sonic. You've recaptured that classic gameplay style of micro-rewards and micro-failures, so I just want to pop in and say "well done" first of all, but secondly I want to comment on difficulty in the Sonic franchise in relation to this game. Sonic is a game series where the core gameplay mechanic makes it tricky to balance difficulty. The basic difficulty lies in managing Sonic's speed and momentum to clear obstacles and achieve a good score by the end from keeping on a quicker path. Later levels have to ramp up the difficulty without interfering with that core mechanic, for example; you can't just throw random obstacles in the way of Sonic. Well, you can, but you end up with zones that are more divisive in the fanbase. Enemy, trap, and puzzle placement requires a lot more care and thought than it would in, say, a Mario game with a one tier gameplay mechanic. The open plan of the demo is perfect for getting that free flying experimental speedrun feel of the classic Green Hill and translating it into 3D in such a way that you're doing much more exploring at the same time, taking it to the next level. (As has been suggested) Later levels might get more difficult by creating narrower paths where you have to, say, for example, use momentum inside some area to go up a wall and dodge some blades. Another alternative that keeps the open plan would be to have harder levels contain enemies that are able to keep pace with Sonic more easily. As you get more difficult zones, you need to make the enemies harder to avoid easily, as is the case in open 3D, and you can't just compensate by covering them all in spikes and filling the screen with them, so you have to increase their "interactivity" instead. Say you have some flying enemy that appears and flies ahead of Sonic (imagine the firefly/hotaru enemies from Stardust Speedway) and because Sonic is "the fastest thing alive" tee em, you should choose the right path to pick up enough speed to get past it/hit it. Traps that fire stuff at you could also get good play, versus stuff that puts you in a narrow corridor to wait for them, or takes things out of your control in some way. You could always have something flat out chase you from behind as well, but then you'd probably need some sort of rear view button, so maybe that's a bit far. Traps like bottomless (or at least lethal) pits will work better in 3D than in 2D, because you can see very far in front of you, so some things that might be considered annoying "artificial difficulty" in 2D function well in 3D. Anything that maintains the integrity of the mechanics for the whole experience is a valid idea. Hope I'm not being disrespectful, by the way. Just wanted to share some thoughts on design philosophy because I'm very excited about this project. Keep up the good work!