A medieval hill castle on a wooded ridge in the Wellenberg above the village of Märstetten, dating from the 12th century and substantially rebuilt in the 17th. Altenklingen is privately owned and run by the Zollikofer family — direct descendants of the merchants who bought the castle in 1815 — and is one of the very few surviving feudal estates in eastern Switzerland still in continuous family hands.
The castle is open for guided tours by reservation only, typically Friday and Saturday afternoons in summer. The Zollikofers run a small farm shop on the property selling estate wine, apple cider, and preserves, which is open during the season without reservation. A short walk through the surrounding woods leads to viewpoints over the Thur valley.