On the tiny Pfalz Island in the middle of the Rhine River near Kaub Germany, stands Pfalzgrafenstein Castle.
It was built for the sole purpose of generating revenue from boats traveling along the river.
While Pfalzgrafenstein is one of many toll castles on the Rhine, its location in the middle of the river on the extremely small island renders it unique.
(Source: http://great-castles.com/pfalztale.php)