After the castle had fallen to the Ernestine line of the Wettin Dynasty, it initially served as a Protestant outpost in the battles of the Reformation era. Beginning in the 16th century it was converted into a mountain palace. The “Heidenbau” wing was erected first, followed later by a French wing attached at an acute angle, the work of Ernestine state master builder Nickel Gromann. With the two richly ornamented oriels on its courtyard side and a spiral staircase with portal, the French wing forms the imposing main building of Heldburg Castle.
In the 19th century the new owners, the dukes of Sachsen-Meiningen, undertook efforts to preserve the castle. Georg II in particular used it as a retreat in the company of his third wife, a commoner on whom he conferred the title of Baroness of Heldburg. Dating to this period are numerous interior remodellings and additions in the Eclectic style, among them the Neogothic baroness’s apartments. On the exterior, the duke’s alterations included adding more stories to the Hausmannsturm and building a terrace with crenellated walls.

Heldburg Castle is a unique testament to the continual further development of a fortified medieval structure and to how a stout castle could gradually be transformed into an elegant palace.
In 2015 the German Castle Museum will open at Heldburg. Visitors will be treated to a representative overview of the development of castles and castle architecture in the German-speaking region, including scenes depicting the everyday lives of the castle’s inhabitants based on their activities in the various rooms. The museum’s largest and most important exhibit is of course Heldburg Castle itself.