Nice, very polished game.
It's satisfying to "pop" those souls, would be even better if there was some sfx when this happens. Art and movement of souls is pleasant to look at.
It would be nice if I could just hold the button and vacuum the souls, I think that's what the 4th upgrade does? It felt very satisfying when that happened :)
I got hooked and played till level 30, not sure if all upgrades have max level, or just the 4th one?
Cheers!