I played normal probably a dozen or more times and my best game I got to level 23 by building lines of machine guns and best gun back and fourth. I was moving forward until about level 20 where my cash flow and the money I was making wasn't covering my losses any longer.
What makes it tougher is that the guns auto-aim at the lowest level unit on the field first which means when the best enemies units start shooting at my defenses, a lone low level unit will wander onto the field and all guns will turn to it letting the enemy units decimate my defenses without recourse. It would be better if you could choose a target or even if the number of mines allowed was based upon the level reached.
I also think the levels were mislabeled. Easy was tough, I would name it normal as it took a number of tries and getting used to how the game worked before you were capable of beating it.
Normal is Hard as I doubt any but the craziest of people are going to beat it and
Hard should be labeled Extreme or something equally as fitting and basically nobody will beat it.
Easy should be a set of levels that the only way to lose is to be stupid..