The castle, Burg Rheinfels, stands – or rather lies in ruins – above Sankt Goar and was founded in 1245 by Count Diether of Katzenelnbogen. After it had been expanded into a fort, it was the biggest defensive complex in the Rhine Gorge and set the standard for castle building throughout the Empire. In the late 18th century, French Revolutionary troops destroyed the fort, after which the complex was used as a stone quarry for other building work. In 1843, Prince William of Prussia, later Kaiser Wilhelm I, acquired the ruin, thus preventing further destruction. Since 1925, the town of Sankt Goar has owned the castle. Today the complex houses a hotel operation and an inn. Also, it houses the town's local history museum.