As Roman, I managed to grab most Germany without much pain. First I used a lot of spies. I made sure I knew exactly where were the enemy troops. I also used forts extensively. Once I chose to attack a city I would use forts and bridges to block enemy stacks, and move to siege the particular city I had selected. As a result, almost every battle I fought was either in a city or bridge... I only had to fight in the forest once or twice.