Conclusion
Here it is, a complete game made in less than 100 events, using careful planning and the power of C2.
You can also think that for a "simple" game as "Asteroid" there is already some quite complex stuff going on at times (the score system for example) which can happen as overwhelming for a beginner.
Any good game designer/programer would tell you anyway that to learn it is good to start with small games/projects.
This one can be considered as a small game as it took me about two weeks (without rushing) to build the capx itself (which is pretty fast/can qualify the game as simple).
I spent a full month on the writing of the tutorial iteslf, so I hope you find it useful.
If you have questions, leave it in the comments or ask in the forums. I or another member of the community should be able to answer it.
There is more that can be done with the project. As I mentioned at some point, you could build up on there, for example having specific behavior happening at specific waves.
I might write another tutorial on how to expand on the project (optimize the code to make "room"/ free more events, add random drops/pickups, etc...).
As I said in the introduction, this tutorial might need to be read a few times with pauses to really sink in.
I hope I have answered some of your questions and displayed useful informations on how to do things and why/how you can use those yourself in your own projects.
A last note, you can reuse this project for learning purposes, you can reuse portions of the code without needing to give credit, but please, don't go and compile the project and try to publish the exported project as your own.
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Updates:
18-09-2012: Fixed the capx that got broken by r103.2 by switching the action "Asteroid: Spawn Asteroid" to "System: Create Asteroid"
Updated the explanation in the tutorial too.
27-07-2015: Revision to support the new Scirra arcade (new example uploaded, new url), implement Local Storage instead of Webstorage (deprecated), adding the "Function" object and a quick presentation of it. Capx made in r210.