It's a lot of trigonometry and he uses java.awt.graphics2d... It's complexicatemened :D. Read through his source code though, I'm sure he can explain it better than me.

I looked, and tried to do the same.;S It's hard. I think you're better than me.;) Could you tell me, what codes i need? ^^

It is rather complicated to try to give you a quick synopsis of what it is doing. I will try to look at coming up with an explanation in the near future. And perhaps if I included some comments in the source code, it would be more easy to follow.

The short answer is likely if you have to ask the question, then you're probably not at a stage where you can implement it. It is indeed impressive and a nice piece of coding, but any 3D or iso-3D like worlds you see in Greenfoot have had the 3D in them coded by hand, it's not something that's built into Greenfoot at all! Of course, this doesn't mean you can't look at the source code, try to gain an understanding of it and learn from it :)