When you first begin the game go with arrow towers exclusively for the first few levels. Dot the first ten as far apart as you can, but still focusing on areas that are well traveled. Do not concentrate the arrows in one area, even if it appears as though it will create a pocket of heavy fire. Go from the front of the path all the way to the Gates of Heaven. It is likely that the no creatures from the first several levels will reach this far, but you will be able to create fortification right from the start. Do not let your gold level get below half what it was at the end of the prior level. This way the interest rate will consistently get a large return, keeping up your gold level. Since you are going almost exclusively with arrows you are not going to be spending as much, which should begin to give you a large surplus after levels nine or ten.