Researchers have uncovered evidence of a previously unknown Iron Age fort beneath the Lilleborg castle ruin on Bornholm, according to the National Museum of Denmark.

The find, dating back to the Germanic Iron Age (approximately 400 to 800 AD), was made during a renovation of Lilleborg in 2010, when Bornholm Museum examined a layer of grain, ash, and charcoal hidden deep under the castle ruin.

Initially, the traces were believed to be from a 13th-century fire at the castle.

However, new carbon samples reveal that the remains are much older, dating back to the end of the 5th century, 700 years earlier than initially thought.

Laurine Albris, research leader on the project, emphasized the rarity of such a discovery, stating, “It is not often that we find traces of prehistoric forts. And it is incredibly unique that the fort is from the 5th century, which we don’t know so much about.”

The existence of a fort on Bornholm during this period is particularly significant due to the scarcity of written sources from the time.

Researchers suggest that the fort’s presence indicates potential tensions in the Baltic Sea region during the Germanic Iron Age.

The research project, named Fortis, is a collaboration between Bornholm Museum, the National Museum, and the University of Copenhagen, and is supported by the Danish Independent Research Fund.

Researchers plan to conduct sample excavations at the castle ruin to further investigate the Iron Age fort, searching for ceramics, postholes, and traces of ramparts.

The discovery also raises questions about who controlled the fort during the Iron Age, as it was previously believed that Lilleborg belonged to the king in the Middle Ages.

Lilleborg, located in Almindingen Forest in the middle of Bornholm, was built in the 12th century and likely destroyed in the mid-13th century.