The remains of the castle can be visited
CastleBonn - Oberkassel
Steiner Häuschen Castle

The castle remains of Steiner Häuschen in Bonn-Oberkassel are a testament to medieval architecture.
History+
The origins of the castle complex date back to the 10th century. By the end of the 12th century, the property was owned by the Counts of Molbach. Countess Alveradis von Molbach donated the estate, then known as “Haistilberg,” to the Cistercian Abbey of Heisterbach before 1210. Around 1217, according to reports by Caesarius of Heisterbach, the castle's tower was destroyed by a lightning strike and subsequently not rebuilt.
Image: http://www.wehrbauten.de/thread.php?threadid=842
Location: 50.7156, 7.1716