The concept was interesting, I really liked the progression system, although I felt it was pretty slow. Also, I think that, if the goal is to kill the spider and not only surviving, you should be able to aim your shots (specially if hitting it gives you money). The skills felt pretty useless, maybe it was because of the aim thing, but I maxed out the player damage and still couldn't get past half of the boss's hp bar. I feel that I would've enjoyed the game so much more if I could complete it in 4-7 waves, but I played more than that and still felt like I was far from the end.
Dev tip: you move faster when diagonaly because the horizontal and vertical movement stack on each other. That can be fixed by simply normalizing the movement vector! ;)