Let's say for the sake of argument I only want to visit them to find and take some nice pictures of external views. I've visited castles and palaces all over the world and to be perfectly honest after a while they all start to look alike to me. But I love wandering around the grounds outside these kinds of places, searching for my favorite exterior views. My home computer desktop background consists of more than a dozen pretty neat shots I've taken myself all around the world (e.g., the Blue Mosque, Chateau du Chillon, the Matterhorn, Stonehenge, etc) and I'd love to add these castles to my collection.
So would I need tickets at all? If all I want is to walk to them from the Fussen train station (and I already know how to get there from München by train and then walk from there to the castle)?