The Co-Cathedral of the Assumption of Mary was built in 1169 as a Romanesque basilica of a single nave on the foundations of a pagan temple of the fourth and fifth centuries, whose remains are still visible at the back of the sanctuary at about 1.5 meters deep. The façade and the south side of the cathedral, in the middle of the eighteenth century was decorated with Romanesque arches and brick niches. In the same period the church was expanded with the addition of two side aisles, altars and other works of art. During World War II the cathedral was severely damaged. The current appearance of the building is the result of the restoration executed between 1949 and 1950.
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Frösöstenen is the northern-most raised runestone in the world and Jämtland's only runestone. It originally stood at the tip of ferry terminal on the sound between the island of Frösön and Östersund. The stone dates to between 1030 and 1050. It has now been relocated to the lawn in front of the local county seat due to the construction of a new bridge, between 1969 and 1971, on the original site.
Frösö runestone inscription means: Austmaðr, Guðfastr's son, had this stone raised and this bridge built and Christianized Jämtland. Ásbjörn built the bridge. Trjónn and Steinn carved these runes.