 , whose ruins are on top of Hohentwiel, was built in 914 using stone taken from the mountain by Burchard II, Duke of Swabia.Originally, the Monastery of St. Georg was within the fortress, but in 1005 it was moved to Stein am Rhein (now in Switzerland), and the Swabian dukes lost control of Hohentwiel.In the later Middle Ages the noble families von Singen-Twiel (12th–13th centuries), von Klingen (to 1300) and von Klingenberg (to 1521) resided here.