Rheinfels Castle is the ruin of a spur castle on a ridge between the left bank of the Rhine and the Gründerelbach valley above St. Goar. After its expansion into a fortress, it was the largest fortification in the Middle Rhine Valley between Koblenz and Bingen and was only surpassed by Ehrenbreitstein Fortress, which is located in the Middle Rhine Valley above the Koblenz district of the same name on the right bank of the Rhine. Rheinfels Castle has been part of the UNESCO World Heritage Upper Middle Rhine Valley since 2002.
The castle was built by Diether V. von Katzenelnbogen in 1245 as a customs castle for ships sailing up the Rhine. According to the only contemporary source, it is mentioned in the Wormser Annalen under the year 1256 that Diether V broke the peace against the citizens of Mainz. The reason was probably the tariffs that had been levied for a long time. The subsequent siege by an army of the Rhenish League of Towns was unsuccessful; this gave the castle a reputation for being impregnable. The Hessian chronicler Wigand Gerstenberg embellished the story further in 1493, which cannot be substantiated by contemporary sources.
In the 13th century, the county of Katzenelnbogen was divided into the upper county in the area around Darmstadt and the lower county with the Rheinfels residence. Around 1360/1370, under Count Wilhelm II von Katzenelnbogen (1332–1385), a large-scale expansion of the main castle took place. Further extensions affected the women's shelter (now a museum) with a round corner tower on the Rhine side and a stair tower on the courtyard side, as well as the mighty shield wall, flanked by two towers, a clock tower and a gunsmith's tower. From 1360 to 1371, the Count built Neukatzenelnbogen Castle, known as Katz Castle, on the opposite side of the Rhine. This made it possible to levy duty on ships sailing down the Rhine (St. Goar double duty).
Source: Wikipedia