"The blue castle overlaps the red castle fief but not vice versa"
I don't understand, how can red not overlap blue if blue overlaps red? Two tiles are always shared, aren't they? I feel like I'm missing something fundamental here.
No. It's not.
Every Castle has it's own "radius" and there can have areas which are in "vicinity" of both castles, and there are also places which are on control of one castle and not of second one.
As vicinity of Castle is not "square", so areas depends of orientation of castle. And red and blue have different orientations.
Try to look, from point of view of blue which areas he controls. Do you see on those areas red castle?
So if red castle scores, blue will not score in the chain, because it is not in his vicinity.
Issue is, that trigger of scoring is not vicinity of scored feature (in this situation, red castle), but it's in other view.
If in castle vicinity is something scored, it triggers scoring of the castle.
(again, from scoring feature to score of the castle, and not from score a castle to score a feature in vicinity)