Älvsborg (also Elfsborg Fortress) is a large sea fortress in Rivö fjord within modern Gothenburg, Sweden. Situated at the mouth of the Göta River, it protected medieval Sweden's only access to the North Sea and the Atlantic Ocean, along with the nearby settlement known today as Gothenburg (formerly named Lödöse, Nya Lödöse, Älvsborg and Charles IX's Gothenburg).[1]