M'well and gameplay wise, I'd suggest to find some more dynamic mechanic.
I don't know, maybe special ennemies vulnerable to only a certain type of turret ? Maybe a "warning" with a direction indicator when a building is attacked also.
Well there is something missing I think it wouldn't be enough in itself. I played the whole game just circling around the base to populate/depopulate the towers. Was funny for 10 minute, then it was only because I wanted to end the game - but it was a bit boring. Despite the graphism looking cool 'n all.
By boring I don't mean "easy", but it's more like... Repetitive much ? I had not to make any strategy to win.
Maybe I'd make turret upgrade more important, remove the fact they can be populated or not ; and make the game about good tower placement and eco priority to get certains upgrade or turrets at good timings ? I don't know.