[Arjuna]

Thanks for asking... I used Apple's SpriteKit framework [1][2]. It's essentially a 2D graphics rendering framework that primarily acts as an abstraction layer for OpenGL ES. In addition, it also offers several other "nice-to-have" features with regard to 2D game development (e.g., collision detection, particles, physics, etc.)

Of course, you could do all of the above without SpriteKit. For example, for all of the rendering, you would simply write everything directly in OpenGL ES.

There are also cross-platform options, like Cocos2d-x [3] and Marmalade [4].

Finally, if you've read this far, you might be interested in reading my general thoughts and feelings from when I finished the game [5].

Cheers!

[1] https://developer.apple.com/spritekit

[2] http://www.raywenderlich.com/category/sprite-kit

[3] http://www.cocos2d-x.org

[4] https://www.madewithmarmalade.com

[5] https://news.ycombinator.com/item?id=8888107