Tower Defense Game, Where to Start?

I have developed and launched a few simple games onto the app store, and am interested in making a Tower Defense game. There is only one problem, I have no clue where to start. To learn how to make my previous games I read tutorials and watched some videos, but for the Tower Defense genre I can’t find anything relevant. 

All resources and experience that you are able to share to help me create my own game are very much appreciated.

Thanks for everything

I have a post on my blog where I had posted a number of demo code pieces. Unfortunately, the one I want to give you is now a broken link, but here’s the blog post:

http://springboardpillow.blogspot.co.uk/2012/04/sample-code.html

The post used to be under this link:

http://developer.coronalabs.com/code/predictive-aiming-tower-defense

I will check at home to see if I still have the code.

The code can now be found in github repos:

Normal predictive aiming to shoot display objects with other display objects:

https://github.com/HoraceBury/predictive-aiming

Physics objects being thrown around, using the same predictive aiming library as above:

https://github.com/HoraceBury/predictive-aiming-physics

I have a post on my blog where I had posted a number of demo code pieces. Unfortunately, the one I want to give you is now a broken link, but here’s the blog post:

http://springboardpillow.blogspot.co.uk/2012/04/sample-code.html

The post used to be under this link:

http://developer.coronalabs.com/code/predictive-aiming-tower-defense

I will check at home to see if I still have the code.

The code can now be found in github repos:

Normal predictive aiming to shoot display objects with other display objects:

https://github.com/HoraceBury/predictive-aiming

Physics objects being thrown around, using the same predictive aiming library as above:

https://github.com/HoraceBury/predictive-aiming-physics