I reached the boss but couldn't finish, unfortunately the very traditional "go to last checkpoint after dying" mechanic contributes negatively here due to how overly prolonged and uneventful the stages are...
In terms of feedback I think other commenters (and your replies) pointed out most or all issues already, so I won't go into too much detail, though I think the fact that the entire game is written in C from scratch is an impressive feat. The visuals are also solid, only held back by the stage design and lack of variety in objects. Despite the shortcomings it's pretty awesome that you were able to implement all this in just 10 days