In the third century AD, the city of Vannes, called Darioritum, acquires the right to fortify. Thus, west of the Gallo-Roman city, a castrum was built. During the Middle Ages, the castrum becomes the centre of the city. Extended in the 14th century, it is reinforced in the 15th century. Over the following two centuries, the ramparts are modernised with the construction of the Garenne buttress. Several medieval gates has been survived, like Tour du Connétable and Château de l'Hermine (former castle, transformed into a palace in the 17th century). Today there is a nice view from the park Jardins des Remparts to the walls.
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.