Once Upon a Castle by Nora Roberts

Once upon a Castle - Spellbound by Nora Roberts:  Calin Farrell is drawn to Ireland where he meets up with Bryna Torrence, a witch who needs Calin to help her in a fight to reclaim her castle.

Castle Doom by Jill Gregory:  Lady Arianne of Galeron is attempting to rescue her brother.  Nicholas of Dinadan comes to do the same.

Falcon's Lair by Ruth Ryan Langan:  Felicity Andrews comes to England in place of her father who has died.  She hopes that her father's friend, Lord Falcon, would accept her.  When she arrives at Falcon's Lair, she is not received very well by Lord Falcon's relatives. Moreover, they claim he is unable to receive visitors.  She ends up being befriended by the castle's ghost.

Dragonspell by Marianne Willman:  Tressalara, princess of Amelonia, escapes her castle when it is attacked.  She ends up living in the woods with the local rebellion group.