Wow... thanks a lot for your long and detailed review.
Originally I wanted to make a small level before the boss (hence the wall climbing animation in the sprites) but my mind went onto something else and this small game stayed WIP for the past 2 years.
If one day I get myself to finish at least the first level (I already have smaller enemies and overall level design) I will use your feedback and bug report to try to polish things as much as I can.
Thanks a lot again. Means a lot to me that someone who take on their time to review something i did that deeply.