That's a good satisfying loop! Well implemented and a different take on the theme :D
I think adding a touch of game feel would make this game come alive. A little screen shake and audible "clank" when you hit stuff, animating the enemies spawning in, and ensuring enemies don't spawn on top of you would make this good game great.
Very addicting! I got everything except lower damage. You really had to grind away at those shield towers!