Rheinfels Castle is the ruins of a spur castle situated on a ridge between the left bank of the Rhine river and the Gründelbach 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 in 1245 by Diether V von Katzenelnbogen as a customs castle for ships travelling up the Rhine. (Source: Wikipedia)