You've got to make use of the interest mechanic inside the game to it's fullest. Using it correctly will make the game very easy (too easy I've been told). A good strategy I found while testing was trying to beat the first waves with as little towers as possible and slowly building up saving towards the +10 interest cap. Then just buying towers every time your money goes over 1000.
Thank you for the kind remarks though. Glad you enjoyed it!