Depending on what plan you use you will likely have a type of tower that you will begin with. The more effective strategies usually rely on arrow towers, especially in the beginning. This is because they cost less, are very effective on early enemies, have a long range, and you can fill up the field with them quickly. Begin placing arrow towers at different spots on the field so that most of it is covered in the first couple of turns. This will help ensure that you end up with the best possible coverage. While you are doing this you should make sure not to ever deplete more than two thirds of the amount of gold you began with at the beginning of any given turn. You need to keep a substantial amount of gold at the end of every turn so that you will get a reasonable interest return.