Göta älv is a place where Danish, Swedish and Norwegian history are woven together. The role of the river grows in importance during the Iron Age and control over it has of course a strategic importance. In the 1250s, the Norwegian king built a fortification on Ragnhildsholmen to protect the town of Kongahälla. In 1308, a wooden castle was built on Bagaholmen, which was named Bagahus, later Bohus.

For more than 300 years, it was expanded and strengthened by Norwegian and Danish kings, mainly Christian IV who visited Bohus and Norway more times than all Danish kings combined. From 1450 to 1658, Danish kings ruled over Norway. Bohus resisted hostile sieges 13 times, but during its heyday, Bohuslän became Swedish in connection with the peace in Roskilde. The Renaissance castle is emptied and now goes to the Swedish king.

In 1789, Gustav III ordered that the fortress be leveled with the ground, but in 1838, Carl XIV said stop to further destruction of the landmark.