Even though I’ve found it to be a bit too easy and not very varied, I’ve found it hard to put it down, so you are definitely on to something here. Some more dangerous enemies (or maybe just a lot more enemies) would not go amiss!
One more tiny suggestion if you don’t mind: you could add a bit of variety to the sounds by randomly pitching them up or down slightly every time you play one.
The visual style is awesome and as a Crimsonland fan I approve!