The construction of the Klingnau castle, originally the seat of the Klingen family, was started in 1240. Until 1269 a manor house stood on the grounds. After 1331 the outer walls were added. In the second half of the 14th century the Bishop of Constance was often a resident in the castle. He ordered further improvements and expansions.
In the late 16th century, the castle, which was the seat of the bailiff from Constance, in such bad condition that the Swiss Confederation demanded a renovation from the bishop. In 1804 the castle went to the newly formed Canton of Aargau, who auctioned it off in 1817. As a result it has been used by various industries, until the 20th century when it was taken over by a foundation.References:
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.