Thanks for your answer but in fact I had hoping that I had missing somthing when I had reading the rules.
I can find plenty of process to place the battlefield but I am surprised that it is not in the rules actually.
I've playing the scenery where my opponent must cross all the table. So, If I have placed some piece of terrain in the middle of the way that he should did taken, that was good for me and very bad for him.
In final way, we have decided that, if he had a good reason to cross very quickly the battlefield, he did be on a road or free way. We have decided that it was no sense if there was something in the middle of his way. If it was the case, my army did should be placed in the middle and not on the sides like in ambush and not a stop battle.
But it is a logic maybe not shared by all.
I had hoped have some explanations about that in the set-up. It had be more simple for our first game if I had not passed so much time to trying to found some help in the core rules.
It is not a problem of optimisation, just an help for the set-up. Not even any indication in the rules. You can understand that I had think this is so strange. And I don't talk about my confidence to read english correctly is shaken .

So I will come back to DBM's rules for finding a way to put some terrains on my battlefield
