The captain of the air ship that saves Tristran belongs to the "Fellowship of the Castle", which seems to be a group whose goal is to help Tristran.
Is this the only mention of the fellowship, or is there something that I've missed? Is the little hairy man a member? Why do they want to help Tristran? Is it related to the Stormhold and Lady Una's curse?
As far as I recall, there were a limited number of times the organization was mentioned:
Tristran shows that he is beginning to care more for the star. They are rescued by a passing sky-ship, on which Tristran's hand is treated and bandaged. The captain of the ship reveals that he is a member of an organization which is later referred to as the "Fellowship of the Castle", whose agents have been working to ensure Tristran's safe return to Wall. The man who gave Tristran the candle was also a member of this fellowship. The ship sets them down on the road to Wall
And the latter mention:
After Tristan's death, there were those who claimed that he was a members of the Fellowship of the Castle, and was instrumental in breaking the power of the Unseelie Court. But the truth of that, as so much else, died with him and has never been established either one way nor another.
The Fellowship of the Castle was also mentioned in The Tale of Sir Lancelot, which might offer a clue behind the meaning in the Neil Gaiman novel. However, it too was seemingly only mentioned in passing, making it difficult to determine its meaning. Sources are also in old English and are difficult to interpret.
Sir Launcelot waxed so faint of fighting and travailing, and was so weary of his great deeds, that he might not lift up his arms for to give one stroke, so that he weened never to have borne arms; and then they all took and led him away into a forest, and there made him to alight and to rest , him. And then all the fellowship of the castle were overcome for the default of him. Then, they said all unto Sir Launcelot: Blessed be God that ye be now of our fellowship, for we shall hold you in our prison; and so they left him with few words. And then Sir Launcelot made great sorrow, For never or now was I never at tournament nor jousts but I had the best, and now I am shamed; and then he said: Now I am sure that I am more sinfuller than ever I was.
and
At last it seemed to him that the black Knights nearest the castle fared the worst, so, as he ever took the part of the weaker, he rode to their help and smote many of the white Knights to the earth and did marvellous deeds of arms. But always the white Knights held round Sir Lancelot to tire him out. And as no man may endure for ever, in the end Sir Lancelot waxed so faint of fighting that his arms would not lift themselves to deal a stroke; then they took him, and led him away into the forest and made him alight from his horse and rest, and when he was taken the fellowship of the castle were overcome for want of him. 'Never ere now was I at tournament or jousts but I had the best,' moaned Sir Lancelot to himself, as soon as the Knights had left him and he was alone. 'But now am I shamed, and I am persuaded that I am more sinful than ever I was.' Sorrowfully he rode on till he passed a chapel, where stood a nun, who called to him and asked him his name and what he was seeking.
Tales of the Round Table: AN ADVENTURE OF SIR LANCELOT (1902)
The first mention, I think, is when Tristran and Hairy Guy ("Charmed") are trapped in the serewood.
The little man shivered. 'I could castle,' he told Tristran, 'but there's no one I could castle with'd be any better off here than we are.'
That's a clear reference to "castling" in chess.
Based on that, I think Gaiman probably invented the Fellowship of the Castle later in the story, based on that earlier reference (itself based on the rules of chess). Charmed has let on that he has a limited set of allies elsewhere who he could instantaneously change places with. Later, Gaiman needed someone to rescue Tristran and Yvaine, so he turned that set of hinted-at allies into a secret society, who apparently have the ability to trade places with each other.
