I just remembered I wrote about some of the worldbuilding problems of Westeros before:
POPULATION DENSITY Technology depends a lot on population density. Without enough people, there will not be enough concentration of resources for certain societal and thus technological development…
military-fantasy.com
Not sure whether I already posted link here, but whatever. Important thing is, it relates to what you wrote about states. Basically, unified states have always depended on rivers: United States managed to stay
united states thanks to the massive Missouri river basin, whereas Europe's fragmentation was also significantly related to geography - both in relation to ocean shores and to rivers and river basins.