Mjälleborgen, Ancient hillfort in Frösö parish, Sweden

Mjälleborgen is a hillfort on Öneberget on Frösön, one of Sweden's northernmost. Its remains form a terrace about 120 metres long and 10 metres wide of earth and stone on the flat north side of the hill; the steep cliffs on the other side served as protection. The structure was held together by an inner timber construction, and traces of two longhouses from the 500s lie within the area.

The fort was built around 300 CE during the Roman Iron Age to control trade routes across northern Sweden. Archaeological evidence shows the site experienced at least two major fires between 300 and 500 CE that destroyed structures.

The name Mjälleborgen comes from the farm Mjälle on Frösön. The fort was probably built by the elite who controlled Jämtland's iron trade, which peaked in the 400s and 500s. The two longhouses inside the rampart date from the 500s.

The remains lie on the flat north side of Öneberget; the rest of the hill is protected by steep cliffs. The terrace is about 120 metres long and 10 metres wide, with no marked paths or facilities. Approach on foot and watch for uneven ground along the earth-and-stone embankment.

The terrace on the flat north side is about 120 metres long and 10 metres wide, while the steep cliffs on the other side served as the fort's defence. Excavations found traces of a palisade around the terrace and of two longhouses from the 500s, held together by an inner timber construction.