Solid game!
I didn't beat it though so take my average gamer criteque with a pinch of salt.
I think the main positives that I took from this game is the art and sound. your choices of colour are clear and contrast well. You're selling yourself short with the screen shots, the game at its most chaotic still maintains a high level of clarity which is a great thing in a side scrolling shooter. The music is eclectic and suits that retro outrun arcade aesthetic so well. I also love that home page and the encorperation of high scores, go ZEBULAN (who will userp this king?). I also think your details in your sprites shows great artistic chops. The dithering effects in the shields deserves a chefs kiss.
My main critiques is the scaling / ramping of certain gameplay elemets. I had maxed upgrades at round 2 and on round 3 didn't care about the other upgrades (but i assume they gave points which is definitely appealing to those pro gamers out there). I also felt that the new enemy introduction was uninteresting. I don't mind the new enemies being powerful slime splitting big versions of the small ones but there was no extra strategy or anything i had to find to defeat them. I think the game becomes post round 2 just becomes more difficult because more enemy which is a little lack luster. That being said it's excusable because that is the arcade sort of thing also.
The highlight for me for sure is that god dang track. I've been listening to it the whole time I wrote this. Fantastic!