The city or stronghold is the castle. The rural region is the countryside. There isn't a castle in a rural region.
That may not be how you've interpreted it, but that is canonically how the BattleMaster universe operates.
Then why have a separate lord for each? An estate with a fortified home, maybe, but the point of the lord is that he was needed to manage lands further out and protect them. If they can't protect their lands worth a damn, then they don't have a purpose.
I mean, we do have that. But it loses the main benefit of cavalry in BM: their charge.
They can still charge once the gate house has been taken, and they can still charge during a sally forth.
And it is not one you will be able to build, because in BattleMaster, a castle is effectively what a Stronghold region is.
It still makes no sense. You would build castles on your frontiers to guard against enemy attack, but the strongholds are cities are frequently towards the core of the kingdom, assuming it's large enough to have a core. Hell, it makes no sense to call most of the various realms independent kingdoms. Maybe petty kingdoms for a few, but a lot of realms seem to be the size of a county at best based on what you're saying.
