Liutfried II of Winterthur is believed to be the presumed builder of the castle, which was first mentioned in 1027. It is believed to have served as a refuge for Count Werner of Winterthur in 1025 when he joined Duke Ernst of Swabia and fought with him against his stepfather, Emperor Conrad II. After the rebellion was crushed by the emperor, Count Werner and Ernst of Swabia fled to Kyburg. In 1027, the castle was conquered and destroyed by the emperor. Werner of Winterthur fell on August 17, 1030 near Falkenstein Castle in the Black Forest in a fight against the men of Bishop Warmann of Constance on the Baar.[3]
After the Winterthur family died out in the middle of the 11th century, their inheritance came to the House of Dillingen through the marriage between Adelheid of Winterthur-Kyburg and Count Hartmann I of Dillingen. Count Hartmann, who fought on the side of the Pope's supporters against the Abbot of St. Gallen, who was loyal to the Emperor, lost the Kyburg in 1079. Despite the defeat, Hartmann maintained his position and became one of the most influential figures in northeastern Switzerland. His grandson Hartmann III of Dillingen, who now called himself Hartmann I of Kyburg, married Richenza of Lenzburg-Baden.
After the Lenzburgs died out in 1173 and the Zähringers in 1218, their lands and sovereign rights passed to the House of Kyburg, which became one of the most important noble families between the Rhine and the Alps alongside the Habsburgs and the Savoys. In 1263, the Kyburgs died out. Their inheritance passed via Anna of Kyburg to the Counts of Habsburg-Laufenburg, who sold most of it to Rudolf of Habsburg. After his election as German king, Kyburg quickly lost importance, even though the imperial regalia were kept in Kyburg from 1273 to 1325. A small part of the Kyburg inheritance was combined under Habsburg administration as the County of Kyburg, an administrative unit that had its center at Kyburg (Wikipedia).