GRRM has been vague about ancient history ('everything we think we know about the Age of Heroes and the Dawn Age and the Long Night comes from accounts set down by septons thousands of years later', 'Past a certain point, all the dates grow hazy and confused, and the clarity of history becomes the fog of legend'), but these suggest to me that During the Age of the Hundred Kingdoms, King Arlan I (the Avenger) swept all before him, extending the borders of his kingdom as far as the Blackwater Rush and the headwaters of the Mander. House Durrandon reached its greatest heights in the epoch that followed.
