It's all downhill after level 5
To be quite honest, the hardest part of the game is beating level 5. There are really only two ways to have a chance to kill the giants (who will make short work of even an upgraded castle). If you have full spell strength and a speed upgrade, you MIGHT be able to win provided the (randomly) appearing enemies don't all come at once. It's especially frustrating to be unable to shoot the kamakaze unit because its behind something larger (or in some cases, the giants). Some people have had luck with the towers, though I don't personally use that until later. Unfortunately, building a moat doesn't help because you will not have enough firepower to kill the giant before it kills you. I think it needs to be tweaked a little so that you don't have to repeat levels 1-4 20 times before "getting lucky" and actually having time to shoot the giants without being massed by other units. Other than that, the rest of the game gives enough time to upgrade.