Really impressive from a technical standpoint. Some others have already mentioned this, but occasionally when shooting the game would freeze for just a split second, but then it went back to normal. Apart from that, it runs great. Some minor bugs I noticed were enemies getting stuck at the edges of maps, and some other enemies suddenly floating in mid-air after being shot. Also, for some reason, the entire game window will occasionally disappear when turning fullscreen off, and the only way I was able to get it back was by forcing the game to quit and reopening it. Looking through the other comments here, I don't think anyone else has mentioned this bug, so it may just be on my end.
The player's score is determined by how many enemies they've killed, which means weaker enemies give exactly the same amount of points as stronger enemies, so there's not much of an incentive to target the stronger enemies. A good way to increase your score quickly is to just avoid the stronger enemies while focusing only on weaker enemies. I'm assuming this will probably change later, but it may be something to keep in mind.
It's looking really good so far. Hope to see more of this in the future.