Ambushes can just be placed in forests, so it would be doable - but dense forests would definitly annoy the player, regardless which faction he plays - he can´t see his troops. Also, I fail to see a great advantage, apart from the +2 forest fighting ability.
But if we would generaly make the winter bonus of the barbarians higher, slow movement rate of the more "civiliced" Factions, and make the game last 4 turns a year - That would strengthen the barbarians, while making it more historically correct that most factions didn´t fight in the winter.
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