The ruins of Bohus Fortress occupy a rocky island where the Göta Älv splits into two branches, just outside Kungälv's old town. Built in 1308 by the Norwegian king Haakon V, the fortress stood at the meeting point of three kingdoms — Norway, Denmark, and Sweden — and became one of the most contested strongholds in Scandinavian history. It withstood fourteen sieges without ever being taken by force, passing finally to Sweden not through conquest but through the Treaty of Roskilde in 1658, which redrew the entire map of Scandinavia.
What remains is a romantic ruin: the main round tower, sections of thick curtain wall, and the outlines of buildings within the courtyard. The tower can be climbed for wide views over the river and surrounding landscape, and the grounds are open in summer with guided tours available. The fortress sits beside the Nordre Älv, and the riverside walk around the island gives good views of the walls from outside. In late July, the area comes alive for the Kongahälla Medeltidsdagar, a medieval festival with a market, jousting, and participants in costume.