Frohberg Castle, Medieval castle ruins in Aesch, Switzerland.

Frohberg is a castle ruin on a rocky ridge at the upper end of the Klus valley, near the old Platten pass road. Its main castle consists of a massive residential block with walls up to three metres thick, built from roughly cut stone, and a curtain wall following the irregular rock edge. A deep natural ditch lies on the west and north sides, an artificial neck ditch on the east, and walls and ditches to the south.

The site was built by the Schaler family in the second half of the 13th century and first appears in records from 1292. The Basel earthquake of 1356 caused significant damage to its structures.

The name Vroberg survives in the local nickname Tschäpperli or Tschöpperli, taken from the field name for the rear end of the Klus valley, itself derived from "Tschopp". The castle was the rearmost of the ruins above Aesch, so walkers on the castle ridge trail reach it first when coming up the Klus.

The ruin sits at the upper end of the Klus valley on the route of the old Platten pass road, the rearmost of the castles above Aesch. The site has not been archaeologically secured, so the terrain is rough and unmarked; wear sturdy shoes. Approach is on foot only, and there is no lighting, so plan to be back before dark.

Unlike many castles in the region, this one was not built to collect tolls from trade routes but emerged from political rivalries between competing local families. This unusual origin sets it apart from typical fortifications of its era.