Olsborg Castle, also Olofsborg, was as a fortified castle 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.References:
Sirmione castle was built near the end of the 12th century as part of a defensive network surrounding Verona. The castle was maintained and extended first as part of the Veronese protection against their rivals in Milan and later under the control of the Venetian inland empire. The massive fortress is totally surrounded by water and has an inner porch which houses a Roman and Medieval lapidary. From the drawbridge, a staircase leads to the walkways above the walls, providing a marvellous view of the harbour that once sheltered the Scaliger fleet. The doors were fitted with a variety of locking systems, including a drawbridge for horses, carriages and pedestrians, a metal grate and, more recently, double hinged doors. Venice conquered Sirmione in 1405, immediately adopting provisions to render the fortress even more secure, fortifying its outer walls and widening the harbour.
Thanks to its strategical geographical location as a border outpost, Sirmione became a crucial defence and control garrison for the ruling nobles, retaining this function until the 16th century, when its role was taken up by Peschiera del Garda.