Sketchy thoughts...

I found your tiles to be confusing here's an example how you can communicate more clearly to the player where the solid edges of gamespace are.
Also just a rough idea to make the spiky turtle thing pop a bit more using higher contrast. you could further make him pop by giving him more saturation.
You have HEAPS of colours within a single layer that aren't doing you any favours so try to reduce your count.

I also notice in ur gif is of the game being played at 2:1 scale, and ur player moves 1 pixel instead of at the 2:1 scale.
This is perhaps due to you needing to scale your art in game.
Might be a small thing many people won't notice but some people would be offended
