Ehrenfels Castle was (re-)built about 1212 at the behest of the Archbishop of Mainz as a defensive work against the constant attacks by Elector Palatine Henry V, who, as Imperial vicar of Franconia, strived to cut down the archbishop's reach. Mainz staffed the castle with Burgmannen and erected a customs post controlling the shipping on the Rhine, supplemented by the Mouse Tower below at the river. Heavily damaged in the course of the Thirty Years' War, the castle was finally devastated by French troops under the command of Lieutenant General Nicolas Chalon du Bléduring the 1689 Siege of Mainz.
The ruin can be reached from Rüdesheim via a hiking trail through the vineyards. The interior however can only be visited in guided tours by prior appointment.
References:Duino Castle was built by the Wallsee family in 1389 on the cliffs overlooking the Gulf of Trieste. It replaced an older castle from the 11th century. Over time, the Wallsee family disappeared and the castle, after having been used as a prison, became the residence of the Luogar and Hofer.
At the end of the 19th century it became the property of Prince Alexander von Thurn und Taxis from the Czech branch of the House of Thurn and Taxis. It remains with the family to this day with his great-grandson Prince Carlo Alessandro della Torre e Tasso, Duke of Castel Duino the current owner. The castle has been opened to the public as a museum and park.