Some interesting scaling on the level design and trippy visuals.
I concur that I'd like to be more actively involved with the game after the level starts. Starting with very little money and then somehow earning money based on how far my units made it towards the eventual goal would let me feel a sense of progression within each level and let me try and adapt my build on the fly.
I also think a second or third type of turret for the humans to use would push people into diversifying their strategies more. It feels like tanking up and just overloading the single-shot turrets ends up winning the round whether it's optimal or not right now. If there were some kind of AOE turret that got blocked by the magic shields or a slow-rotating rocket launcher that had trouble following fast units then those would give you a lot more options in terms of level design.