By Eric Haines (a.k.a. Eric5h5)
This tutorial shows you how to add an enemy object to the simple 2D Shooter project located here, which requires at least version 2.0 of Unity to work. It's not an in-depth look at actually creating such a game, but rather an example of working with a Unity project and some additional scripting. Thus it might help relative beginners with Unity to see an example of how a project can be structured to be easy to work with and extend, though it does assume some basic familiarity with Unity's functionality, and the scripting discussion assumes basic knowledge of programming concepts and terminology. The code in the project that already exists isn't covered here, but it's reasonably well commented, in an attempt to explain the thought processes behind creating a 2D game in Unity. (Not all thoughts are guaranteed to be 100% correct or sane. ;) )
If you get stuck or confused, you can use the project located here as a reference for the tutorial. It's the same as the project from the above link, but it also includes everything from the tutorial up to but not including Part 7, section 4 (wouldn't want to take away all the fun!).