My fuzziness about geography and post-medieval history means that I was never quite sure what the lowlands were. Belgium mostly, I thought muzzily, with bits of southern Holland and possibly northern France, forgetting Luxembourg entirely. I have ancestral French reasons for not wanting it to be Belgium itself, and personal ones as well; but I suppose it might be an AU Belgium in which people wash occasionally. Then again, late 17th century, nobody washes, so the point is moot.