interesting concept. I like the graphics too.
Overall I think the game need some tweaks to be more balanced. The crowded enemies is an interesting idea but it's kinda tricky to kill them before they come (maybe it's intended, like you have to be more strategical but if so, I think either you need to go faster or they need to be slower so that would allow you to fallback more easily / avoid them / etc).
there is also some physics glitch like some can traverse walls, bullet hit wall even you are too close to a wall. Also the AI of enemies is pretty basic.
cheers