Indeed - further to Dickie's tip, a good start is crucial. If you put all your money in the bank before the first wave, and click Next Wave twice, the interest from the second wave will give you 95 gold - enough to build a purple tower right from the start. As long as you place that purple tower on the second narrow strip (down and left of the bank) quickly enough so that it can fire at creeps on both paths, you will not leak any creeps on those first two waves. One purple tower will enable you to survive for quite a long time, just putting your bonus money into the bank to earn interest.
More fun... as long as you don't mind being a) frustrated and b) addicted