The control is a huge problem. Once the protagonist gets moving it takes too long to come to a stop or turn around. It seems even the enemies have better maneuverability than the player. The current max speed is fine, but the acceleration and friction need to be magnitudes higher.
That was pretty much the main problem. I think there are some issues with the level design (like having a snake camp the top of a ladder), but they might not turn out to be much of an issue once the controls are fixed. The graphics look nice, but the assets look familiar.
The graphics don't render correctly when running on my computer's integrated graphics, and I needed to use the NVIDIA processor. For troubleshooting in the readme you should suggest right-clicking the .exe and selecting "run with (NVIDIA) graphics processor", in case other players have similar problems.
Animation looks very nice and fluid, and I appreciate the effort put into making minor animations, like teetering on a ledge. The music and backgrounds look great as well, although I had a hard time seeing some platforms in the tomb levels. The sound effects could use some work though (particularly the axe swings)
For a game that describes itself as Sonic+Castlevania I was surprised at how good the combat is. Grounded melee attacks don't halt the player, the little hop in an aerial attack helps for platforming, the slide was useful in dodging boss attacks, it's a great package. The downward slam attack definitely feels off without some sort of landing lag though. And again, some better sound effects could probably make attacks feel more "punchy".
Most of what I didn't like about the game is, appropriately, most of what I don't like about 2D sonic games. The acceleration is too low, angled jumps on slopes feel weird, levels get repetitive and overstay their welcome. It could be a preference thing and sonic fans probably won't mind, but it bothered me. Still the levels could probably be shorter or have a greater variety of enemies.
Biggest problem: this breakable platform that surprisingly dropped me down to a lower level I'd already been to in the first tomb level without any way to get back. That felt unfair. I think it's unreasonable for the player to be expected to consider the layout continuity of the cave like that... especially since the room before that somehow had a bottomless pit on the second floor. If it's supposed to be a fast way back to a locked door after getting a key, they player should come across the key first.
Still looking forward to see how this game turns out as it's pretty good so far. I found a debug-ish menu that let me see the other characters and they all look like a lot of fun.
One last thing: I found that sliding off a platform will cause the character to run way faster than normal upon landing. I'm not sure if that's intentional but it was fun to play with so I'd actually suggest leaving that in.