Tunaborgen

Tunaborgen is a medieval fortress ruin in Österskär, featuring a square tower, a bastion structure, and courtyard walls positioned beside the entrance of an ancient waterway. The remains are protected by a wooden roof and secured by gates, with the interior structure clearly showing medieval construction methods.

The fortress was first documented in 1291 by Archbishop Nils Alleson and served as a strategic defense point along the shipping route to Uppsala. Its position on this important waterway made it a key location for controlling trade traffic in the region.

The excavated ground floor displays three brick-framed windows and supports for ribbed vaults that show how medieval builders worked in Sweden. Visitors can observe how people then used stone and brick to create structures designed to last.

The ruin is protected by a wooden roof and secured with gates, with interior access requiring prior arrangement with local authorities. Visitors should check on-site how to explore the structure and whether guided tours or viewing times are available.

Ground radar surveys in 2014 detected additional walls beneath the surrounding grassland, revealing that the medieval complex was larger than the visible ruins suggest. These buried structures show the original site was considerably more extensive than what remains above ground.