Creating an iPhone app - Math Legend

I really enjoy posts on the creative process or evolution of a product from initial thought to completion.  Below you'll find some images of my most recent app, Math Legend.  The app follows the design of games like Guitar Hero and Rock Band and offers a unique way to practice math problems. 

I accidentally left the app available for free for a couple days and something like 4,000 grabbed it.  That's exciting.  One mother wrote me to say her girls were having fun practicing math.

The first screenshot shows a proof-of-concept I created after thinking about how this game might feel.  I played around with this on a real device, letting my imagination run with my fingers.  I also wanted to make sure the device could handle the 2D rendering without being jerky (I ended up doing some work trying to get things to run smoothly, but the demo reassured me that things would be in the realm of doable).

The second image shows a playground of icons and fonts.  I experiment with many looks while coming up with my icons and logos.  The designs that became final are towards the left.

The other images are from the final product.