Nassau, originally a county, developed on the lower Lahn river in what is known today as Rhineland-Palatinate. The town of Nassau was founded in 915. Dudo of Laurenburg held Nassau as a fiefdom as granted by the Bishopric of Worms. His son, Rupert, built the Nassau Castle there around 1125, declaring himself "Count of Nassau". This title was not officially acknowledged by the Bishop of Worms until 1159 under the rule of Rupert's son, Walram. By 1159, the County of Nassau effectively claimed rights of taxation, toll collection, and justice, at which point it can be considered to become a state.