This would make for a pretty good mobile game. The upgrades felt good and there was some interesting tension between the ghosts which demand your attention and the need to hold for upgrades. I really think it could deliver the same experience in less time (i.e. be shorter), because I think after level 7 or 8 I felt like there wasn't much difference and the waves didn't get much harder either (clicking acts as AoE against the ghost waves so I think it would have been more challenging if they were spread out and numerous).
But overall pretty good, the limitation was done well and the art/sfx/vibe was good too.