to answer your questions:
1), 2) The simulator used in Aseba Playground, Enki, is actually a 2D simulator. The height we give to objects is only used to display them in 3d, but in the simulation there is no notion of height, the gravity is not calculated… So there cannot be slopes, holes, bridges…
3) The only way to display color on the ground is by using a texture.
4) Unfortunately, sounds are not implemented in the simulator, I am really sorry

Thank you very much for your anwer and the example.
It was very helpful.

I still have some questions and I wonder if you could further help me.
In a "world" of Playground, is it possible to:
1) MAKE A HOLE? I tried to create a wall with negative value of h, but it didn't work out.
2) PUT THYMIO ON A WALL AND MOVE IT, ON IT? (in order to simulate Thymio on a table).
3) DRAW A 2-D COLORED AREA?. (in order to make the playground texture, instead of importing a suitable png picture).
4) HAVE SOUNDS IN PLAYGROUND WITH VPL? You see, my effort is run activities with my students with a virtual Thymio, as buying 11 robots for school is not possible. Kids like colors and sounds and that, would make lessons with virtual Thymio more attractive.

Thank you again for your help
George Psoinos

We do not have a good documentation for this, so I made an example playground file featuring the different possibilities with explanations. I hope this helps!
Download the example