Here is a tip, A good way to 'rank up' without buying it is, going to a easy map yu like, play on med/hard mode, what ever you like best, beat the main waves, then when your done hit 'keep playing'.
A good 30 levels of pure EXP. AND there pretty simple because by then you all read have ALOT good stuff and cash!
Really fun game! One of the better tower defenses! But i do have 1 complaint, They save some of the better stuff for people who buy them with kong points, this is FINE but they like to 'show it' alot to you when your dont want to see it. IE: the 'buy a ranks nao!' when a new tower is unlocked, or when you die. "try this weapon it will make the game more easy for you for a small $$$!" Other then that, good game! 4/5