The Therapist Gamer

Pixeljunk: Monsters is a tower-defense game on PS Vita and PS3. You control "Tiki Man," who walks the path between his cottage and the monsters. You can build a tower on each tree, and each tower gains experience based on how much damage they do to the monsters. When they get enough experience they will automatically upgrade. Alternatively, you can upgrade the towers by spending rare gems, or by gradually increasing their experience by standing over them with Tiki Man. 

The game is fairly straight-forward but very well designed and balanced. I'd say it's probably the best tower-defense game I've played (but Kingdom Rush is about as good). It's not for babies, though - the game is pretty tough!