A squat, circular stone fortress on a former islet at the eastern edge of the Kvadraturen, completed in 1672 to guard the approach to Kristiansand's harbour. The thick limestone walls, the deep moat (now mostly dry), and the central round tower make it one of the most distinctive buildings in the city.
The fortress is now used as a venue for concerts, exhibitions and summer events; the interior is open to visitors free of charge during the day in the summer months. From the ramparts there is a good view across the harbour to Odderøya and the entrance to the fjord.