Alvsborg Fortress

New Älvsborg, so-called to distinguish it from the earlier fortress at Old Älvsborg, is a sea fort on the island of Kyrkogårdsholmen within the urban area of modern Gothenburg, Sweden. Situated near the mouth of the Göta River, it protected what was at the time of its construction Sweden's only access to the North Sea and the Atlantic Ocean. Construction began in 1653, and the fortress remained in service until 1869, though it only saw significant action in one conflict, the Great Northern War.

Source: Wikipedia