I remember reading somewhere that a castle owning clan can still siege a fortress (register and not get xp penalty on seige field) but not claim it. Is this true and is it only possible for regional fortress linked to the castle?
I dont think castle holding clan can have a fort. We had fort and owned rune for about 10 minutes. The moment we owned Rune, our fort was lost to us. We had to retake it afterwards.
No what I mean is if you have a castle, can you still register to siege a provincial fort (not care about owning it - just want to get no loss xp when pvping inside)
Yes the clan owning a castle can siege a fortress and they can "take it" however they wont be able to officially take it, it will just revert to NPC mode.
It allows castle owning clans to prevent clans which are going to siege your castle from owning a fortress for fast entry into the castle's siege field.