for the visual, i think it's not a good idea to do 90 degree top view. I can see the reason is to make the 2d characters rotate without extra art work. But then the player can only see the character hair. If you really want to minimize workload, i suggest you at least make left and right direction (like fighting game) so that player can see the characters sideway.
Also, it's quite hard to feel the range a tower can cover, it will be better if it shows a circle when player hover a tower spot. And just like other said, it's better to have fast foward button, and more towers/ enemies, although i can tell you run out of develop time.