That's a good thought. Castles feel more like scenery than something I really care about right now.Checkpoints/fortifications. Since the roads would be important (and the most effective way to transport large armies), forts/castles/checkpoints could be built on them (or simply have the roads pass near preexisting castles). This would discourage armies from bypassing forts/castles
Your point about checkpoints made me think of the Roman Public Courier System (Cursus publicus), which had many checkpoints and stops scattered around the empire. You could hook quests/events into these kinds of mini-locations like "Help tax collector of <Kingdom> to do XYZ" or have new kinds of roaming parties - taxmen and administrators - as rare/difficult but valuable targets that travel between cities and checkpoints of a kingdom which you could rob (or protect for influence).