Overall very cool concept! There are some improvement that can be made on the art side but very cool idea.
There are still two things that annoyed me:
One is that i wasn't able to buy towers or click buttons sometimes... (those are just technical issues)
But more importantly i would make the map be more like a tower defense game map so everything is on One screen (no camera pan) and make the roads go over each other so one tower can shoot the same enemies twice.