If you have enough defenses to keep the creeper at bay, and you are making more power than you are using, wait until you have some energy stored up before building anything else. This way you can expand without draining power from other things.
The concept of the game itself isn't very new, but what makes this excellent is its focus on playability and functionality. Instead of, say, little animated marching armies everywhere, it has zooming, and panning and saves. The only thing I would suggest would be to make it so that the map should wrap when panning.
The balancing should be better, and I wish I could drag things directly from the shop to a character, instead of having to move it to the inventory first.
The only problem with this game is that it is way to short, and doesn't need a lot of skill. The king was a lot easier than the final town, which I was never able to beat, even though I had no trouble with any other 5 star towns and missions. I completely forgot about the upgrades, and made it through the entire game without buying any. I hope you make a sequel to this.