Robber barons, or robber knights, were wealthy German land owners who violated the structure of tolls on the Rhine either by charging excessively high tolls or by operating without authority from the Holy Roman Emperor. During the Great Interregnum (1250–1273), the number of such tolling stations guarded by castles exploded in the absence of Imperial authority. These castles are so thick along the Rhine that three or more can be seen at the same time.

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