Hey!
Here are my 5 cents about the game :)
What I like:
The gameplay is solid and challenging and the effects look really good! When hitting walls the bounce effect is visual and the trail helps to facilitate that feeling of movement really well. Also aiming and shooting feel very well in place. Being trapped in that tiny breathing square also adds a good sense of stress to the overall feeling. Enemies have different HP and ammo is restricted yo one has a good tactical aspect in there too.
I also like that it works with simple shapes and polygons and builds the aesthetics more by effects than by fancy textures etc. I always like simplicity in games. Also it is not easy to get a good high score and seeing the leaderboard right now, I need a lot of practice it seems ^^. For me it is super hard to reach 1000.
The game loads up super fast in the browser by the way and the music fits the theme well. Good job.
Some remarks:
Just one tiny thing. The enemy bullets' color can be optimized imho. The bright purple has basically the same brightness than the bright player square background and that is a bit hard to read. (see this monochrome image without saturation)
That is critique on a very high level, because I do not what else to criticize :)
Overall:
Very good entry here and thanks for the time and effort you have put into this!
Keep it up!