"Feature," maybe. If it gave the castle a quality of warmth, that to me implies that the whole castle became "warmer" because of the tapestry, when what I think is implied here is that the tapestry was the single warm thing in an otherwise cold, possibly gloomy or forbidding atmosphere. Characteristic is the same, and sounds awkward to my ear besides.
A bit of warmth, or a touch of warmth, is closer to what I think the sentence means. But those are rather "cozy" words and not very appropriate here.