Nice concept. And I love the enemy designs! Obviously, there's more you can do with this idea but what you were able to implement over the weekend is great. Good job!
Also, if you're thinking about sticking to your art style, I'd suggest trying out vector art. Vector art stores images as lines instead of pixels, so your illustrations will have smoother lines that can be exported at any resolution. Adobe Illustrator is good if you have it but I'd recommend Inkscape as a great open source vector art program.