The last stop for the week is the island castle called Pfalz, or Palatinate.
The description translates as follows:
The Palatinate, situated in the Rhine at Kaub, a highly original building, whose main pentagonal tower dates from the middle of the 11th Century. The little castle was built for the purpose of revenue collection.
Presumably, this location made it easy for tax collectors to intercept commercial boat traffic on the Rhine.
This is number 133 in the series.