The covenant I'm currently working on is hidden in a regio atop a hill near an Italian city state. It can look out and see the city but to people outside the regio it's just a wood covered hill top.
The covenant has a basic ringwork set of defences. Does it need a 'Castle' Hook given the Duke of the City can't see the covenant and get upset about the fact he doesn't control it?
If the fact that you have a castle does not come into effect as a generator of stories, it does not work as a hook. Hence it does not need to count as a hook even if you do have a ringwork castle