Lahneck Castle 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.