Karlsborg Fortress (Swedish: Karlsborgs fästning) is situated on the Vannäs peninsula in Karlsborg by lake Vättern, the province of Västergötland, Sweden. Construction on the fortress began 1819 to realize the so-called central defense idea adopted by the Swedish military after the Finnish and Napoleonic Wars. The site was chosen by Baltzar von Platen in connection with the construction of Göta kanal.