Baldenstein Castle was the family castle for the Knights of Baldenstein, the first of which was mentioned in 1246. The central tower which now forms part of the castle was built around the same time. Very little is recorded about the Baldenstein family and by 1289 the castle was owned by the Freiherr von Löwenstein. Before 1289 he lost the castle in a war with the Freiherr von Rhäzüns, but in the peace treaty of that year received it back after giving Schwarzenstein Castle to Rhäzüns. In 1349 it passed to the Übercastel family. Wilhelm von Übercastel planned to expand and strengthen the castle, but was prevented by the Bishop of Chur, until he granted the Bishop certain rights. Originally the castle consisted of a rectangular bergfried. During the Late Middle Ages an administrative and residential wing was added to the west.
Around 1400 the castle passed to the Freiherr von Stein. Through marriage it then passed to the von Ringg (later Ringg von Baldenstein) family in 1453. In 1562 Luzius Ringg sold the castle to Jakob Ruinelli. The Ruinelli family lived in the castle for several generations. Jakob's grandson, also named Jakob, accompanied Jürg Jenatsch to Rietberg Castle in 1621 to murder Pompeius Planta during the Bündner Wirren. He, in turn, was stabbed to death in 1627 in a brawl between drunken officers. After his death the castle was inherited by the Rosenroll family. During the 16th and 17th centuries the castle was further expanded with a new, larger wing replacing the medieval residential wing. During the same time, the medieval ring wall was replaced with a new crenelated wall.
In 1738 the castle was acquired by the Salis-Sils family, who owned it for almost half a century. In 1782 it was inherited by Francesco Conrado, from Chiavenna in Italy. Francesco became a member of Senate of the Helvetic Republic after the French invasion of Switzerland in 1798. Eventually his descendants changed their name to Conrad and still own the castle. In 1877 the residential wing was mostly destroyed in a fire. It was rebuilt to its present appearance in the following years.
The Petersberg Citadel is one of the largest extant early-modern citadels in Europe and covers the whole north-western part of the Erfurt city centre. It was built after 1665 on Petersberg hill and was in military use until 1963. It dates from a time when Erfurt was ruled by the Electors of Mainz and is a unique example of the European style of fortress construction. Beneath the citadel is an underground maze of passageways that can be visited on guided tours organised by Erfurt Tourist Office.
The citadel was originally built on the site of a medieval Benedictine Monastery and the earliest parts of the complex date from the 12th century. Erfurt has also been ruled by Sweden, Prussia, Napoleon, the German Empire, the Nazis, and post-World War II Soviet occupying forces, and it was part of the German Democratic Republic (East Germany). All of these regimes used Petersberg Citadel and had an influence on its development. The baroque fortress was in military use until 1963. Since German reunification in 1990, the citadel has undergone significant restoration and it is now open to the public as a historic site.