As in he was a vassal of the king of France, so logically his conquest would have resulted in gain for the French crown too. Did William declare independence, taking his French lands with him? Or was it more complicated than that?
Or was it more complicated than that?
Take it from someone who hangs around the Medievalists: it's always A Lot More Complicated Than That, whether or not we're using the F-word. Though judging by the threads I'm about to link, it's not that much more complicated. More can always be said on the matter, so if anyone would like to comment further on William the Conquering Bastard and how he related to the King of France, please don't let this post stop you! For the meantime, OP, these previous threads may be of interest to you: