You can play on any difficulty, though the recommended is H/M I believe. Playing VH campaign will make the AI go berserk on you and it is nigh impossible to have peace with any neighboring factions. VH battle difficulty makes the AI's troops have much higher morale, armor, etc..., which means that levy troops can sometimes defeat troops much better than them. Very annoying. And of course, you could always play on easier difficulties than the recommended, which will make the game...well... easier. But AFAIK changing the difficulty doesn't mess with the game script.