Pretty cool little game! I do love me some simple pixel games :D
It starts off kind of frustrating. The wheel is slow, you don't have a lot of nodes to fire with, and you tend to die pretty quickly. I was thinking to myself "this needs some kind of auto-fire or AoE" when I finally leveled the rotation speed and it unlocked the mouseover firing. It would probably be nice to have the next spell hinted at, rather than having it hidden until you invest.
That said, once you DO unlock the mouseover, it becomes trivially easy as you just rotate your cursor counter-clockwise on the nodes to spam beams in all directions. The unlocks further trivialize the game, but I imagine that's something you could easily fix given more time to develop.