Disputed Lands – maybe, but there’s no textual evidence. Likely depends on terrain – are there any hills or fords or other geographical features which would make a fortress useful? Or is it a featureless plain where mobility and flexibility are key and fortresses just limit you?

Lower Rhoyne – far more likely, although again, no direct textual evidence. Forks in the river, fords, bends in the river where the speed of the current changes, etc., rivers offer far more locations where one would want a fortress.