You are going to be there for over 12 hours? Well, sure, you can; but why? If planning on arriving that early, are you staying in Fussen the night before and with a car? Otherwise you would have to walk from Fussen; i dont think the buses run that early. If you have a car, you need to park in one of the private parking lots in Hohenschwangau. I think the cost for a car is 10€. That early (or late) in the day, you will have to walk up the steep hill (about 2km) to the castle and about another 300 m uphill to Marienbrucke bridge. Because it is situated at the top of a hill with sheer cliffs on most sides, it is not possible to walk around the whole castle outside the walls. But there is a paved road leading to the entrance along one side of the castle.