yes, borders of territorial holdings absolutely existed, i didn't mean to imply otherwise.
But as you say they were a great deal more fluid. They did not usually imply the kinds of things modern borders have a strong tendency to (immigration controls, language and culture, etc). Nor was the reality of them largely viewed as immutable and above reproach.
Indeed not. Immigration control and passports did not arise until the first world war, in fact. Border crossings were quite open until then.
But culture was not insignificant. They did not define borders (and some might argue they still don't do to this day) in any way, but they guide monarchs whereto increase their realm's size.
But in a way, while culture didn't define borders, borders did define cultures. Where they speak French or German is largely attributable to the duchies and kingdoms where they spoke French and German.
The arbitrary borders of Europe can largely be attributed to this.
But as you say they were a great deal more fluid. They did not usually imply the kinds of things modern borders have a strong tendency to (immigration controls, language and culture, etc). Nor was the reality of them largely viewed as immutable and above reproach.