Frohburg Castle, Medieval castle ruin in Trimbach, Switzerland.

Frohburg Castle is a medieval ruin perched on a hilltop with remnants of stone walls and building foundations dating back centuries. The scattered remains allow visitors to trace the layout of the fortress and see how space was organized within its defensive walls.

The site was settled in the Bronze Age and again in Roman times; in the 10th century an aristocratic family settled on the ridge and from the late 11th century appeared in documents as the Counts of Frohburg. They built a wooden settlement first, then stone buildings within the perimeter wall, and controlled lands between the Aare and the Rhine until their line died out around 1367. By then the castle was already largely abandoned and was later used as a stone quarry by nearby farmers.

The name Frohburg, not Froburg, was confirmed by a Germanic linguistic study; locals used it until the early 20th century. The counts took their title from the castle. Excavations after 1970 uncovered an iron smelting furnace and evidence of bone working, crafts tied to Jura raw materials.

The site is accessible year-round via marked hiking trails and requires no entry fee. Bring sturdy shoes since the terrain is uneven and there are no facilities at the ruins themselves.

Excavations after 1970 uncovered a small iron-smelting furnace inside the walls, exploiting iron ore from the Jura. Bone-working is also documented, and the castle had at least three water cisterns. A Germanic study confirmed the name is Frohburg, not Froburg.