Isenburg, Castle ruin in Isenburg, Rhineland-Palatinate, Germany

The Isenburg is a Romanesque hilltop castle ruin above the village of Isenburg in the Westerwald, where the Ommel, Wiebel and Iser brooks meet the Saynbach. Its elongated plateau measures about 70 by 30 metres and was divided by a cross wall. The square bergfried, 9 metres on each side, stands in the south-west corner on a rock spur, not on the attack side; parts of its second and third storeys and a high entrance about 6 metres up survive.

The Isenburg was first documented in 1103 in a charter of Archbishop Bruno of Trier, and was probably built around 1100 by Rembold and Gerlach I of Isenburg on land held by Fulda Abbey. Because the family soon split into several lines, it became a Ganerbenburg shared by related branches, with houses such as the Kobern, Wied and Frauenhaus. It was still inhabited in 1625, but after the extinction of the Nieder-Isenburg line in 1664 an inheritance dispute left it to decay; in 1771 the southern half of the bergfried collapsed.

The Isenburg was a Ganerbenburg: several family branches shared it, each with its own house, such as the Kobernhaus, the Haus Wied and the Frauenhaus, also called Haus Grenzau. Tower, gates and bridges were used jointly. In the village below, the Alte Porz gatehouse, restored in 1959, has housed a local museum since 2008.

The ruin stands on a 250-metre spur above the village of Isenburg, where the Ommel, Wiebel and Iser streams meet the Saynbach. The site is owned by the Prince of Wied in Neuwied, and since 2005 the Freundeskreis der Isenburg association has worked with Rhineland-Palatinate's heritage office to secure the remains and reopen them to the public. The village lies on the B 413; the Alte Porz gatehouse in the village centre houses a local museum.

The bergfried stood not on the exposed northern side but in the southwest corner on a rock spur, from where the Sayn valley could be watched in both directions. Its square ground plan measures 9 m per side, and the surviving high entrance lies about 6 m above the ground.