Forces of Nature (on sale at $0.99) by Mean Bean Games is a game that will catch your eye immediately because of the art. There's something fairy-tale like and compelling about the hand drawn art that reminds me of an animated film I saw when I was a kid, The Iron Giant. Another thing common between Forces of Nature and The Iron Giant is the fact that they're both sending a strong environmentalist message.
Mother Nature's champion is Sprout, a feisty little seedling who plants various defenders to fend off heartless robots that have taken over his world. Sprout's mission is to protect the giant tree from land and air units. Water is his main currency for planting seedlings which can be fixed (tower defense type) or moving. Sprout does damage on his own, and strategically dashing behind enemies can even score combo points.
At the heart of Forces of Nature is a side-scrolling castle defense dressed up rather beautifully and quirkily.