Hohentwiel Fortress is a former fortress on the volcanic headland of Hohentwiel in Hegau, near Lake Constance. The rocks of the phonolite vent tower 260 meters above the town of Singen, located at the eastern foot of the mountain. With an area of nine hectares, the fortress, accessible to visitors, is the largest castle ruin in Germany. Over 80,000 people visit the site every year.
Geologically, the Hohentwiel lies in a unit that encompasses the Hegau and western Lake Constance. Sedimentation formed the various layers of the surrounding area. Volcanic activity in the region began with the formation of the Alps. A remnant of this is the massive chimney of the original Hohentwiel, filled with deck tuff. On its eastern rim, the phonolite rises as another volcanic testimony.
Above the visitors' path between the arsenal (fortress above) and the service building (below), 190 m² of TECCO® System G65/3 with P66 spike plates were installed using GEWI-32 and titanium 40/16 nails.