The Heuneburg is an oppidum, a fortified settlement, near Riedlingen. Today it’s an open-air museum known as the Keltenerlebniswelt Heuneburg (Celtic Experience World) and is protected as a cultural monument (Kulturdenkmal). It opens only in the warmer half of the year, is closed on Mondays unless it’s a public holiday, and shuts completely over the winter.
Without a car, you get there on a regional bus line that runs only on demand. The one thing to plan for is food: it’s not clear whether there will be any on site, because the catering at the Keltenerlebniswelt is currently being put out to tender. It makes sense to bring your own.
