Gamasutra has uploaded an informative post-mortem for Infinite Interactive's Puzzle Quest. CEO and designer Steve Fawkner is basically a master at mixing casual and hardcore gameplay, so if you're keen to learn a few of the ingredients of his magic recipe, this is a good place to start.
The article also has some great insights into the minds of casual gamers. Take this, uh, gem, from the end of the piece:
* Some of the standard RPG conventions were absolutely mystifying to casual gamers, such as matching purple gems to level up.
I know I don't blink an eyelid at terms like "level up", "skill points" or "man girdle", so it's hard to imagine how a casual gamer would feel faced with such esoteric concepts.
There's loads more on how Infinite pitched the game and the challenges it faced with different platforms, as well as the steps the developer took with the original prototyping.
Postmortem: How Puzzle Quest Saved Infinite Interactive [Gamasutra, via Sumea]