Hohentwiel is an extinct volcano in the Hegau region of Baden-Württemberg in southern Germany The mountain lies to the west of the city of Singen and 20 miles ( 30 km ) from Lake Constance .
Hohentwiel is an extinct volcano in the Hegau region of Baden-Württemberg in southern Germany . It is west of the city of Singen and 30 km from Lake Constance .