Burgruine Rosenau, Medieval castle ruin in Königswinter, Germany.

Burgruine Rosenau is a castle ruin in the Siebengebirge range near Königswinter with surviving ring walls made of trachyte stone. The walls reach heights of around six meters in places and sit surrounded by forest.

The first written record dates to 1222, when the castle owner adopted its name for his own. After the monastery purchased it in 1243, the fortification was demolished within a few years.

The castle belonged to a chain of fortifications with Rolandseck, Godesburg, Drachenfels and Wolkenburg that guarded the southern flank of the Cologne archbishopric. Its name passed to Dietrich von Dorndorf, who called himself Dietrich von Rosenau from 1222. The site later served as a quarry for Heisterbach Abbey.

Several hiking trails lead to the ruin, with a moderate loop route from Ittenbach providing direct access. The site is most accessible during warmer months when forest paths are dry and easy to walk.

The 1243 sale contract explicitly granted the buyer, Heisterbach Abbey, the right to destroy the castle, an unusual clause. A second contract followed in 1249 after a daughter-in-law of Agnes von Rosenau objected, and the abbey demolished the castle around 1250, likely reusing its stones in monastery buildings.