I dunno if underdeveloped is the right word. Seasonal might be the better description – if you look at Robb gathering the banners in AGOT, or Bran’s harvest feasting in ACOK, the North definitely gets together at Winterfell to politick and intrigue and propose marriages.
So my guess is that Winterfell’s court in winter is massive, as Wintertown turns into a city filling up with nobles and commons alike. And because everyone’s basically trapped indoors and sharing one another’s company all the time, I expect that you get very intense relationships, both positive and negative. I’m thinking it’s very reminiscent of classic Russian literature, but with more broadswords.
But I think distance works against having a court during the other time of the year. Rather, House Stark seems to have turned to dynastic marriages and tours of the provinces instead.
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To answer your other questions:
- I think Catelyn didn’t bring any ladies with her because of the war.
- Northern ladies I think it’s mainly the distance issue.
- Squires probably are de-emphasized due to the lack of the knightly tradition.
- As for fostering, Ned seems to have taken against it due to his generation’s experience.