Dear Wordwind,

What an excellent idea!

No dictionary to hand so I can't check (should that be against the rules anyway?), but the first word that comes to mind is garderobe - but I suspect it is pronounced as two syllables, not three. The privy was, in mediæval times, called a necessarium, but that seems to have one or two too many syllables. I think reredorter has some connection, maybe that was that a separate building for the same purpose (it connects in my mind with monastries rather than castles). Can't come up with any more - but a good headscratching exercise.