Ragnhildsholmen brings together a medieval castle ruin and archaeological remains. The surrounding area has been inhabited since at least the end of the Bronze Age and includes one of Bohuslän’s largest burial grounds, with remains dating from the late Bronze Age to the Viking Age.

The castle is mentioned in Icelandic annals as having been built by Håkon Håkonsson before 1257. It had a main wall around a square courtyard, buildings for the garrison and stores, a tower on the east side, and a protected entrance on the south. A rampart and ditch surrounded it. The site was in use for less than 100 years. The website says it was probably destroyed by fire and that its stone was used for Bohus Fortress.

Visitors can explore the ruin, walk among the habitats and watch for birds. The website describes guided explorations of the castle ruins, bird walks and tours of the burial grounds, but gives no dates or locations for these activities. It does not provide opening hours, an admission price or a suggested visit length.