Idea was quite interesting. I liked mouse based switching instead of number pads other games used. And that small mini map was amazing addition! At the beginning I didn't see it and I was a bit frustrated for not being able to know where are enemies in other layers. Since you can switch into an instant death. But mini map fixes that.
For improvements I would suggest making everything a bit bigger, maybe I am just too old but it was a bit hard to see enemies and even your own projectiles.
Overall I did enjoy the concept, it has potential to grow into something bigger, so keep it up and do not stop creating games!