You may have played GemCraft before — it’s a pretty popular Flash game that’s been available for a while now. It’s a tower defense game, though the new layer here is that your towers are actually powered by gems, which allows you to do things like move them around for various monster waves and combine and upgrade gems as you go.
You can also use those gems to cast spells and abilities, which will slow monsters, attack them directly, or power up your defenses in other ways. The iPhone version has 32 levels to play through, graphics updated for Retina Display, and full Game Center integration.
GemCraft is well-balanced, colorful, and a lot of fun, even for the somewhat well-traveled Tower Defense genre. You can play it for free in Flash form at the link above, play a free iOS version, or download the full game for just US$1.99.