Student or Learner
In a medieval castle, what's a linen cellar?
Maybe the room where to weave?
However, I suspect that even mediaeval housekeepers would have preferred to keep their linen in cupboards or chests in an well-ventilated room, not in a cellar which would be underground and probably damp.
As bhai suggests, a place in which to keep linen. But if this is part of your games-based story, it is an additional indication that the person writing it has no real knowledge of mediaeval life or language.