Robert IV of Nassau

Robert IV of Nassau (German: Ruprecht) (died ca. 1240) was an early member of the House of Nassau. The House of Nassau would become an important aristocratic family in Germany, from which are descended the present-day rulers of the Netherlands and Luxembourg.

Robert IV was the second son of Count Walram I of Nassau and Kunigunde of Ziegenhain (a town now part of Schwalmstadt, Hesse), daughter of Count Poppo II of Nidda. On his father's death in 1198, he became co-Count of Nassau, with his brother, Henry II.

Walram I had received the Königshof Wiesbaden from Emperor Frederick I in reward for his support of the emperor in the conflicts of 1170-1180. Nassau’s possessions in this area were expanded around 1214 when Henry II received the Imperial Vogtship (Reichsvogtei) over Wiesbaden and the surrounding Königssondergau, which he held as fiefdoms. In about 1200, Robert, together with his brother, began building Sonnenberg Castle on a spur of Spitzkippel peak in the Taunus above Wiesbaden. This was intended for protection against the Archbishopric of Mainz and its vassals, the Lords of Eppstein, who held the lands bordering Wiesbaden. However, the cathedral chapter of St. Martin in Mainz claimed Sonnenberg as their own. To settle the dispute, Nassau paid 30 Marks to the cathedral chapter in 1221 to acquire the land of Sonnenberg Castle. Henry was also forced to recognize the sovereignty of the Archbishops of Mainz over Sonnenberg, taking the castle as a fief of Mainz.

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