dude.. i start off with just three of the land cannon things and upgrade them.. i dont put anything else up until level 7 when i get fire elment...then i put two fires up and delete the other two cannons...then let the two fire cannons take as many levels out as they can until i need another fire cannon.. the more money u have in your bank the more interest you get..it builds up fast if you dont spend it so dont put anything out in the grass thats not needed till later levels.. i upgrade by going fire, interest, interest, cold, interest... no need for the rocket cannon thing.. interest is better. then about the ending levels in the 30's is when you pack your grass full of towers..