Burg Pfalzgrafenstein (Pfalz Castle)

From www.travelswithfriends.com:

The Pfalz Castle was an ancient toll booth of sorts, and today is probably the most photographed castle on the Rhine. Built in the 13th Century, this castle is on the river bed near the town of Kaub. It, along with the Burg Gutenfels (now a hotel), made the town of Kaub one of the wealthiest on the river. When boats would approach, the castle would raise chains to the opposite bank of the river. If the boat paid a toll, it could pass without a problem. If tolls were refused, the boat would be seized and the crew was put on a raft in the well of the Pfalz castle. Boats that tried to bypass the tolls would be ripped to pieces from the chains and the strong current.