Olsborg Castle

Olsborg Castle (Swedish: Olsborg), also Olofsborg, was a fortified castle located in Tanum Municipality, Sweden, in the province of Bohuslän. It is located on a steep cliff, and might previous to later use been an early hill fort. It was constructed in 1503 or 1504 by the squire Nils Ragvaldsson from Åby, after a recent Swedish attack on Viken. Most of it was destroyed shortly after, when the commander of Bohus Fortress Otto Rud attacked on Christmas night 1504.

After the turmoil created by the dethronement of the Danish king Christian II, the Swedes returned, seized the area and reconstructed Olsborg's fortifications. The brief Swedish rule of northern Bohuslän was administrated from Olsborg by the nobles Lars Siggesson (Sparre) and Ture Jönsson (Tre Rosor),between 1523 and 1525. In 1525, the administration was moved to Karlsborg Castle, south of Hamburgsund. During these years, the castle defenses were improved, especially in 1526. During Christian II's attempt to retake his throne, the part of Bohuslän under Swedish rule was attacked, and both Karlsborg and Olsborg were destroyed by Norwegian forces in December 1531.

Decades later, several attempts to rebuild the castle was made by Sweden. Eric XIV in 1564 and Johan III in 1569 and 1570 both tried, but the attempts were unsuccessful and the castle remained in ruins. The ruins were still clearly visible during the early 20th century, when they were excavated by Wilhelm Berg, who also excavated other castles in the province, such as Ragnhildsholmen Castle and Dynge Castle.