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.
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Presumably, this location made it easy for tax collectors to intercept commercial boat traffic on the Rhine.
This is number 133 in the series.