The Frohburg is the ruin of a rock castle in the municipality of Trimbach in the Swiss canton of Solothurn and is one of the largest medieval castle ruins in the Jura Mountains.
The castle ruin is classified as worthy of national protection, and is therefore at the highest level of monument protection awarded in Switzerland.
The place was populated as early as the Bronze Age and then again during the Roman period. In the 10th century, an important noble family in the Olten and Zofingen region settled on the Jura ridge. In the immediate vicinity of the castle, the castle and followers of the castle lords opened up new settlement land by clearing. The family has appeared in the documents as the Counts of Frohburg since the late 11th century. The family derived its name from the castle name. It dominated a large area of influence between the Aare and the Rhine in the old regions of Aargau, Buchsgau and Sisgau until the 14th century.
After a first modest settlement with wooden houses, the counts erected a large number of larger structures within the surrounding wall of the castle over the centuries. In addition to a count's house, service buildings and workshops were also part of the assembly. Of particular interest is a small smelting plant for iron, which was exploited as an important natural raw material in the Jura. In addition, the processing of leg has been proven to be another specialty trade. The castle had at least three water cisterns.
In the course of the 14th century, the Counts of Frohburg lost their privileged position, and the individual buildings in the family's ancestral castle were abandoned over time. When the counts died out around 1367, there were probably no more permanent residents in the complex, which has since fallen into ruin and has been used as a stone supply by residents of the agricultural settlements in the area.
First investigations of the site before the middle of the 20th century and scientific excavations after 1970 provided rich information about the settlement history of the castle.
A detailed Germanic investigation carried out with the archaeological studies has shown that the castle name really means «Frohburg». This is how the place was called in the area until the early 20th century, and only in later decades did the incorrect spelling Froburg spread here and there due to an incorrect approximation of the name to the Old High German word group of fro (= Herr).
Today the facility is one of the popular tourist destinations in the Olten region.