Current questions and answers regarding the map editor.

I meant that I don't see any parameter in the arena properties to set any object there.
I did it that way, but the wall is not displayed visually.

To set any object:
AdvMapDesc => Combat Arenas => + => Desired arena => Double-click the left mouse button on the arena resource.

A map will open, and in the very center there will be a zone of 10*10 cells; you can place ABSOLUTELY ANY OBJECT there.