Hohentwiel, 78224 Singen, Germany
An extinct volcano with the ruins of a castle on its top. Hohentwiel began forming, along with the chain of volcanoes in the Hegau region, about 7–8 million years ago. Hohentwiel Castle, whose ruins are on top of Hohentwiel, was built in 914 using stone taken from the mountain by Burchard II, Duke of Swabia.
Location of Hohentwiel