I’m afraid that most people who worked in castles didn’t have unions, yet. Though it would be kinda funny if you’re investigating a haunted castle and it turns out the ghost isn’t some 14th century person, but just some tour guide who died in 1980. Though tour guides are probably not unionized in Germany, not sure which modern castle-related professions would be unionized.

Though most people who worked in castles also weren’t royalty (debatable if royalty “worked” at all), which might come as a disappointment if you expect that castles are only haunted by noblemen and -women, which I’d say is what’s typically depicted in fiction and legends.

“A ghost is haunting Germany – a unionized ghost”

It’s a life cycle thing: You start as an apprentice, then you are a tour guide and finally you are what the tour is about.