Groothusen was built on a warft - one of those long, artificial dwelling mounds the Frisians piled up against the North Sea before dikes existed. In the early Middle Ages this mound rose at the edge of a tidal bay, and ships could sail right up to the village. That changed everything for a tiny dot on a marsh. Groothusen became a trading centre, a meeting place in the Frisian Emsigerland, the seat of a provost who oversaw twelve parishes. Three castles defended the wealth that the tides brought in. The bay has long since silted and been diked away, leaving Groothusen stranded inland - a quiet farming village whose importance now lies entirely in what survives from when the water was its highway.

What you see today began around 1490 as a single long, two-storey hall built by the Beninga family, one of the East Frisian chieftain clans whose feuds and alliances ran this corner of the coast. Around 1550 the hall sprouted wings on either side, turning it into a three-winged manor house. Then in 1700 the right wing was torn down to make way for something the Frisians had perfected by then: a Gulfhaus, the enormous barn-house hybrid where livestock, harvest, and family shared one giant pitched roof. Walk around to the back of the Osterburg and the centuries peel away - you can still read the original 1490 hall in its bricks. From the front, a hallway added in 1900 muddies the view. The building is honest about its layers; it just makes you walk around to see them all.

Inside, the Beninga descendants who still own the Osterburg have kept what their family kept. Furniture from generations of dinners. Weapons from feuds that history has otherwise forgotten. A library, items of Old East Frisian culture, and most striking of all - a wall of ancestral portraits spanning five centuries. The painted Beningas stare across the room at each other in their ruffs and stiff collars and 19th-century military coats, a family tree rendered in oil. Tours run by appointment, arranged by telephone, which feels appropriate for a place this stubbornly continuous. The left wing has been quietly converted into guest rooms; bed and breakfast in a chieftain's hall, with lime trees outside and a moat below.