I find that it makes no difference which place I choose to fight, it's nearly always the same. Bridges are an exception. For the most part though your army doesn't inhabit a large enough area for having a mountain pass really be all that strategic or much of a difference to fighting in an open plain as opposed to the tile between two mountains. Even if you choose to fight in an area and feel it's good enough for duking it out with your opponent, when the battle loads, you find your soldiers in the most awkward of places. And it isn't as if you have the time to skirt up a mountain pass and hold that, there just isn't the time or the terrain detail for that kind of thing. So basically I see no reason to choose the ground for the battle aside from the meagre area you can maneuver around in one the battle commences.
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