1980-95 Historical Fiction, Castle, Romance, Mystery, Murder, Stables

Young woman lived with her ailing father. Ran into stranger who ended up living in a castle. (Duke maybe?) She’d heard mysterious things about the castle and its inhabitants. Although afraid of him, she ended up living/working there. She was manipulated in trying to get her father to move to the castle. Mysterious things happened. Disappearances, hidden passageways. There was a murder. Possibly stable hand helped her in her quest to find answers. She eventually followed the main character through the woods to find out what he was up to. Father eventually moved in with her.
Following details could help but if you have any suggestions and these don’t fit, go with your gut.
Possible title “The Mystery of ? Castle” but not entirely sure. The paperback cover might have had a small white house with a path through green fields. Message me for more details if needed.