You should (based on the season you arrive) travel from Koblenz Mainz by boat. Be aware that there are no bridges along that part of the river, but ferries (schedules!).
@@theadventurousheart1782 & TWIMC - the building in the river at 04:02 is a historic toll station ("Die Pfalz bei Kaub")... de.wikipedia.org/wiki/Burg_Pfalzgrafenstein en.wikipedia.org/wiki/Pfalzgrafenstein_Castle