| On easy mode since they come from left to right I just made a row of towers above and below and put one on the end to make them go around. Only upgraded the towers near the middle of the screen so that you could concentrate fire on the flying ones.
I did it a second time and used about approx 30 towers. I upgraded only around 7-8 but used a lot of the 5gold towers to make my maze. This wound up much easier than the first win. | 