What position have you, or would you for the sake of discussion, give to the Castellan? A baron whose barony has seen better days? A baronet or knight of the realm? Maybe just a military officer acting as a steward for a noble ruling in absentia?
I always opted for the latter -- basically a captain of troops and royal henchman of the "Realm of the West." You probably already read this, but I just pulled this off of Wikipedia, and seems to be a good discussion of the term:
"A castellan was the governor or caretaker of a castle or keep. The word stems from the Latin Castellanus, derived from castellum 'castle'. Usually, a castellan combined the duties of both a majordomo (responsible for a castle's domestic staff) and a military administrator (responsible for maintaining defences and protecting the castle's lands). This was particularly the case if there was no lord resident at the castle, or if the resident lord was frequently absent. In France, castellans (known in French as Châtelains) who governed castles without resident nobles acquired considerable powers, and the position actually became a hereditary fiefdom."
An idea I just had would be for the true lord to be away, on some high adventure or off to the crusades -- it might be an adventure hook, later on.
When I started tossing this around today, I was thinking the latter was the most appealing just for the adventure hooks that it could create. I was thinking about the possibility of a greedy noble who has suddenly taken some intrance in the place showing up after years and trying to evict the Castellan. Or maybe the noble is actually a fake, an agent of the Shrine of Evil Chaos.