Oh, I am relieved. I didn't think you could do that for some reason. That's very nice

Yeah, actually polymorphism is explained in the bgt language tutorial. It is a beautiful thing once you figure it out. I wish bgt subported abstract classes though. They seem pretty cool, because they force you to conform to a syntax. Basically so you don't miss something when you define your subclasses. That's what I learned about in java.

I'm not sure if you'd necessarily need anything abstract classes provide that you can't get with interfaces, unless it's variables?

Where is the inheritance in your Purebasic example?  And why is Purebasic coming up, anyway?
The whole point of polymorphism is not variables, it is class methods.  You cannot do polymorphism without very interesting pointer tricks in languages that do not provide built-in support for classes.  You can do giant ugly switch statements and if trees based on a type field, but the whole point here is that inheritance and polymorphism get rid of said switch statements and if trees, splitting your code into something that's manageable and can be considered in isolated chunks.  They also let you add new fields to your subclasses, i.e. Enemy versus NamedEnemy.  And now we're both off topic, I guess.