Ruins Altenberg, castle ruin in Füllinsdorf, Switzerland

Altenberg is a ruined hillside castle on the Büechlihau ridge above Füllinsdorf. Its centre is a tower house measuring about 9 by 15 metres with walls 1.6 metres thick. A 35-metre stretch of curtain wall runs north-west of the tower, with a double garderobe built into it; a residential tract with kitchen stood at the north end and the gatehouse in the southern corner.

Built shortly after 1000, probably by a high noble family to extend power over the border between the Kingdom of Germany and the Kingdom of Burgundy, the castle is undocumented in written records. A fire around 1050 destroyed part of it and it was rebuilt, but it was abandoned shortly before 1100 and partly demolished. Its lands later belonged to the Lords of Schauenburg, whose original seat it may have been.

The name Altenberg gave way in the 17th and 18th centuries to Filisberg, a name possibly sharing its origin with Füllinsdorf itself. The site is now a class B Swiss cultural property of regional significance, and the excavated finds, including a piece of gold and silver jewellery, are what remain of its inhabitants' world.

The ruins stand on the steep, wooded Büechlihau ridge above Füllinsdorf and are reached on foot; the walls were conserved and the site opened to the public in 1987. The layout runs lengthwise along the slope, with a marked neck ditch on the northwest side and a section ditch with a forward rampart on the southeast.

Excavations uncovered fish bones from coastal regions, revealing that inhabitants had access to long-distance trade networks. This discovery shows connections to the wider medieval world far beyond the local area.