Lahneck Castle (German: Burg Lahneck) is a medieval fortress located in the city of Lahnstein in Rhineland-Palatinate, Germany, south of Koblenz. The 13th-century castle stands on a steep rock salient above the confluence of the Lahn River with the Rhine, opposite Stolzenfels Castle, in the district of Oberlahnstein. Its symmetrical plan, an oblong rectangle, is typical of the later castles of the time of the Hohenstaufen. The pentagonal shape of the bergfried is rare for castle towers.
The castle became well known in England by the death of Idilia Dubb in June 1851. While on holiday with her family, the 17-year-old girl mounted the abandoned castle's high tower, when suddenly the wooden stairs collapsed behind her. Nobody heard her crying and calling from the tower, because it was surrounded with an insurmountable wall, 3 metres high. The last sentences in her diary: All I know is that there is no hope for me. My death is certain. ... Father in heaven, have mercy on my soul (with two hearts drawn below). She was only found after years in 1860, her diary hidden in the walls some weeks later.
Lahneck Castle was built in 1226 by the Archbishop of Mainz Siegfried III of Eppstein to protect his territory at the mouth of the Lahn, where the town of Oberlahnstein and a silver mine had come into the possession of the Archbishopric in 1220.
The castle chapel, dedicated to Saint Ulrich of Augsburg, was built in 1245, in the same year the first burgrave took up residence in the castle.
In 1298, King Adolf of Nassau was a guest at the castle, shortly before his death in the Battle of Göllheim against King Albert I of Habsburg. In order to avenge him, the Burgrave of Lahneck, Friedrich Schilling of Lahnstein, participated in a conspiracy against Albert. The castle was stormed in 1309 and Friedrich Schilling was executed.