Add like wind and birds ( or something) that might get into your way, or every time you fail and your rocket hits the ground something should have a percentage to break.
all and all i give it a 5/5. i disliked the arrow tower it got boring and repetitive. i do like how you don;t have to buy new units just upgrade them. Which is nice. i do wish though you had more to do with your hero such as talents or armor or something.
Though, i just defend with my horde of goblins in the beginning...then pull the main guy with my skill then when he gets to me i use my skill again and it;s over.