Natural defenses will always be a problem in nation RPs. The people who take up on mountainous terrain will always be harder to invade, people in cold terrain will always be harder to siege. I suppose it all balances out though. If you're in a frozen tundra, food shortages would probably plague you. Same with mountains because of a lack of farm land. The poison grass would probably work the same. Can't have livestock, but your enemies can't feed their horses.

Just my two cents.