Festung Hohentwiel, Medieval fortress ruins on volcanic mountain in Singen, Germany.

Festung Hohentwiel covers nine hectares on the phonolite summit of the Hohentwiel, a dead volcano rising 260 metres above Singen. A curved grey stone wall on the left encloses a small round courtyard with a few trees; low walls with narrow openings run right towards a tall, square, flat-topped stone tower. Further right, roofless grey walls line the edge of a large grassy area.

The fortress was first documented in 915 and experienced five sieges during the Thirty Years' War. French forces destroyed it deliberately in 1801 to eliminate its military threat.

The Hohentwiel Festival has run in the fortress grounds since 1921, when performances moved into the ducal castle courtyard; a jazz festival followed in 1975 and is still held annually. Miles Davis played here in 1990. Today the festival lasts one week in July.

The hike to the site starts from Singen via marked trails, with the route taking 30 to 60 minutes depending on your entry point. Sturdy shoes and windproof clothing are important since the path is steep and the hilltop location means exposure to weather.

The fortress covers nine hectares, making it the largest castle ruin open to visitors in Germany. Visitor numbers have exceeded 80,000 every year since 1990, peaking at 126,520 in 2002.