yet some another ideas for further improvement
the game itself is great and it's getting better and better by every update. here some another clues for further improvement:
1. In single mode the enemy doesn't send troops to attack you. instead he just increases his defense. you should fix that, because (even on impossible difficulty) it's very, very easy to destroy him (just build some troops with high range - get much money by destroying a tower or turret - finish him by using strikes) or becomes endless (you are out of money to buy troops but the enemy doesn't attack you even if your base is completely defenseless).
2. i don't know if it's possible, but please create a multiplayer mode. it would be awesome to seek out human enemies.
thanks and keep up the good work! :D