The Petersberg Citadel (also known as Petersberg Fortress ) is an originally Electoral Mainz , later Prussian city fortress of the 17th to 19th centuries, located in the center of the Thuringian state capital of Erfurt It is considered one of the largest and best-preserved of its kind in all of Europe and was built in 1665 on the orders of the Elector of Mainz and Archbishop Johann Philipp von Schönborn as a bastion against the city in the New Italian style. In the further course, as the northernmost fortress, it was intended to protect the electorate from attacks by the Protestant powers. The strategic importance of the citadel was later recognized by Prussia and then France , who briefly annexed it at the beginning of the 19th century . With the Congress of Vienna in 1815, together with Erfurt, it finally became part of Prussia and remained so until the German empire was founded in 1871 used as a fortification. It remained a central military location in the region during both world wars and in the post-war period.From 1963 the site was partially open to the public. From 1990, the state of Thuringia and the city of Erfurt carried out large-scale renovations.