Istein castle was a rock castle built by the bishops of Basel in the 11th and 12th centuries. It was first time mentioned in 1185. The castle was stormed in 1410-1411 by the Basler troops. The modern fortress was built in 1900-1914 and destroyed after 1945.
St. Vitus Chapel, partially ruined, is one of two original castle chapels built in the 12th centurya. The current small chapel, which was built after the destruction of the castle in 1650 and destroyed by the French occupation forces in 1947, was rebuilt in the 1980s due to a citizens' initiative.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.