Quote Originally Posted by Mechstra
You can only have one guild per city.

Knightly Orders and the Woodsman's Guild are the only guilds that can be established in castles - they can also appear in cities, or at least the Orders can. Can't speak for the Woodsman's Guild, but I'd be surprised if it couldn't.
I think I had a Swordsman Guild established in one of my Fortresses (Nottingham Fortress as English).

Does the game list the guilds possible for Castles in the Show Buildings list?