Survivor of no fewer than 14 sieges, Bohus Fästning is a hulking ruin on an island in the Nordre älv, near Kungälv. Founded in 1308 by the Norwegian king to protect Norway’s southern border, the fortress was inherited by Sweden at the Peace of Roskilde in 1658. Its substantial remains include a remarkable round tower, Fars Hatt, where unfortunates were once imprisoned for witchcraft and heresy; you can peer into the lightless dungeons, and climb the battlements for a lofty view.
Tourist information for the area is available at the fortress. To get here, take the Västtrafik Grön Express bus from Gothenburg to Kungälv (30 minutes, every 10 to 15 minutes); get off at the Eriksdal stop and walk the remaining 500m.