What is here is fairly solid - the first level(s) are a decent tutorial, the art is nice, and the level design is fairly solid (I see that Door to Before!) It could definitely do with a couple more levels though, and a minimal tutorial for the checkpoints might be nice (alternatively, you could remove the need to interact with them at all and just have the player come back at the nearest one to where they died.)
Here's a video of my full playthrough: