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Regarding adding an arena, from what I read in that thread, they only change the visuals.
Is it possible to add a new arena, even if it's a copy, but of a different size, that will work?
And I didn't understand how to put it into a duel.
Added 15 minutes later
I think I figured out how to put it in, but how do you change the size of the battlefield?
Larger "arenas" exist in Heroes 5.5, but this is done through scripts or by editing the source code (I'm not sure).
In general, the video tutorial wasn't about "large" battlefields, but about creating your own arena and adding it to the list of arenas in duels.