Bohus Fortress, or Bohus Fästning, is a fortress from the 13th century at Kungälv in Sweden. Construction began under king Håkon VI of Norway, who named it "Bagahus", later shortened to "Båhus". Strategically located where Göta älv divides to surround the island of Hisingen, it was for a long time the central installation for defense of southern Norway against Sweden. The fortress passed from Norway to Sweden by the Treaty of Roskilde in 1658 together with the province of Bahusia, Båhuslen or Bohuslän, which literally means the "Fief of Bohus".
After Denmark-Norway had lost Bohus a new fortress, Fredriksten, was constructed in Fredrikshald on the new border against Sweden. Bohus had served its role as a border fortress but after being captured by Sweden, it lost its strategic value to the Älvsborg Fortress at Gothenburg and the new fortress Carlsten that was being erected at Marstrand.
