Former duchy, Holy Roman Empire .
Located on the Rhine River , the area now lies in the districts of Düsseldorf and Cologne , Germany. In the 11th century the counts of Berg acquired Westphalian lands east of Cologne; these were incorporated into a duchy in 1380. Berg became a leading iron and textile manufacturing centre in the 17th–18th centuries. In 1806 Napoleon made it a grand duchy in his Confederation of the Rhine . Following the Congress of Vienna in 1814–15, it became part of Prussia .