Hi! I asked this to @warsofasoiaf a while back, and I was hoping to get your thoughts as well. Since, unlike the North, the Riverlands have no natural defences, did Robb ever have any chance of maintaining and defending that part of his kingdom in perpetuity? Even in the best of circumstances, wouldn’t whoever ends up ruling the south eventually conquer the Riverlands as soon as a slightly weaker king ascends the Northern throne?

This is a bit of fanon that annoys me. The Riverlands has natural defenses: they’re called rivers. It doesn’t have defensible borders or boundaries, in the sense that the Riverlands has lands outside of its rivers to the west, south, north, and east, and that rivers are easier to cross than the Neck or the Mountains of the Moon, but that’s not the same thing. 

However, as history has shown time and again, when Riverlanders pursue a strategy of defense-in-depth as opposed to perimeter defense, they can easily deal with invaders: this is true whether you examine Arrec Durrandon’s campaign that led to the Battle of Fairmarket, the downright miraculous campaigns during the Dance of the Dragons against the Westermen, the Reachermen, and the Stormlanders, or the Battle of the Fords during the War of Five Kings. 

So if the North adopted an effective administration (with support from the political community of the Riverlands) that built up a riverrine navy, used the rivers to gain the superior mobility of interior lines and to force any invader into fighting at chokepoints where you have a huge defensive advantage, and was willing enough to retreat back to the inner lines of the Trident when necessary, I think the North could hang onto the Riverlands, even if pressed, for an extended period of time.