Defender's Quest: Valley of the Forgotten feels like it was designed by people who really understand what makes the tower-defense genre enjoyable, and how it can be improved. Elements normally found in role-playing games are present here, and there's an intriguing story -- yep, this one is dangerous.
If you're vaguely compelled by the trailer, be sure to try the in-browser demo, which allows you to export save data. Defender's Quest is being sold for $6.99 and that includes all future game updates. The developers were even nice enough to offer a coupon code (DESTRUCT). Use it this month to knock a dollar off the price tag.