The tour (suggested by Rick in his guide book) sounds great -- includes key sights and Hop on Hop Off bus. BUT, I can't figure out how to use it to book a reservation for a specific day and time. And how (if possible) to add the guided tour inside the castle. Any guidance would be very much appreciated!
The bus company have an allocation of tickets outwith the public booking system. They guarantee you entrance, and will reserve a time slot for you from their own allocation.
So you are at the mercy of the bus company for what time you get.
There does not seem to be a way to then add the guided tour, as that is normally sold as a package with entry.