Alt-Falkenstein Castle, Medieval castle in Klus, Switzerland

Alt-Falkenstein is a medieval castle in the Klus gorge near Balsthal, in Switzerland, set on a rocky spur above a narrow valley passage. Several buildings and walls make up the complex, fitted tightly between steep rock faces on either side.

Bishop Burkhard von Fenis of Basel, rewarded with the Buchsgau landgraviate by Emperor Henry IV for loyalty during the Investiture Controversy, had the castle built on a rock in the Jura transverse valley to guard his territory's border. Count Rudolf I of Falkenstein (attested 1224–1251) is considered the builder, with construction around 1250; the castle was first documented in 1255. In the early 15th century the impoverished nobility sold it to the city of Solothurn, whose patricians enlarged it and used it as a seat for their bailiwicks.

The castle holds a local history museum with archaeological finds and weapons from different periods. The collection also covers how people lived and worked in the Thal valley over the centuries.

Guided tours run from April through October, giving visitors access to the fortress and museum inside. The paths are steep and involve several sets of stairs, so sturdy footwear is a good idea.

The castle sits so tightly between the cliffs that rock walls close it in on nearly three sides. This was no accident, as the builders chose the spot precisely because the terrain itself did most of the defensive work.