[1] The citadel was built on Petersberg hill, in the north-western part of the old town centre from 1665, when Erfurt was governed by the Electorate of Mainz.

[6] The citadel was built on the site of a medieval Benedictine Monastery and the earliest parts of the complex date from the 12th century.

[7] The former lower barracks (German:Untere Kaserne) building is now used to house and administer archives of the Stasi Records Agency.

The city was first mentioned in 742 in a letter from St Boniface to Pope Zachary, when the Catholic Diocese of Erfurt was established.

The earliest archaeological find from the Middle Ages on Petersberg hill is a coin dating from c.850, from the time of Lothair I, Holy Roman Emperor from 817 to 855.

The Swedish king, Gustav II Adolf, hoped to build a fortress on Petersberg hill, but he died in 1632 and the plans never came to fruition.

[5][7] The formal first construction phase lasted from 1665 to 1707, following an order from Johann Philipp von Schönborn (1605–1673), Archbishop-Elector of Mainz and Bishop of Würzburg and Worms, to build the citadel.

Von Schönborn had recently conquered Erfurt and wanted to build the fortress to demonstrate his power to the local population, in order to suppress any ideas of rebellion.

[5] On 4 November 1813 Prussian, Austrian and Russian forces drew in on Erfurt and sought to get the French to surrender and leave Petersberg Citadel.

[12] In June 1873 Emperor Wilhelm I gave orders for the Petersberg and Cyriaksburg citadels to be pulled down, but due to lack of funds this was mostly not done.

After World War I, as a result of the Treaty of Versailles in 1919, there was a gradual abandonment of military facilities during the Weimar Republic period.

Petersberg was used for garden allotments and by the civil police, who had a detention centre, garages, workshops and stores on the site.

They were kept in a police detention centre originally built to accommodate up to 60 prisoners, but at its peak 241 internees were crowded into the facility.

[5] Some war damage was sustained, including the 12th century Leonhardskirche (St Leonard's Church) which was totally destroyed in an air raid.

[10] At the end of World War II, Erfurt was liberated by American forces in April 1945, and handed over to the Soviet administration on 3 July 1945, as agreed at the Yalta Conference, held in February 1945.

In 1944 a transit camp for displaced people had been established in the Defense Barracks, and this continued operating under the occupying Soviet administration after the war.

The Stasi, whose prison and district headquarters were immediately below the citadel, had warehouses and workshops on the site for their motor pool until 1990.

Most buildings added since 1868 were removed, most earlier buildings, including the 12th century St Peter's Church, were restored or stabilised, ditches were cleared and walls and bastions were repaired, the horchgänge (listening passages) were cleared, and the 1832 wood-fired fortress bakery was brought back into working order.

At the time the citadel was first built there were no trenches or moats beyond the wall, so the Horchgänge were constructed so that soldiers could walk through them to listen for anyone trying to tunnel into the fortress.

It was designed by Antonio Petrini and bears the coat of arms of Johann Philipp von Schönborn (1605–1673), the Archbishop-Elector of Mainz.

Until the end of the 13th century, emperors of the Holy Roman Empire and German kings often held court in the church.

[11] In November 1181 an Imperial Diet was held in the church, presided over by Holy Roman Emperor Frederick I, known as Barbarossa.

There Barbarossa's cousin Henry the Lion submitted to the emperor's authority after several years of opposition and war, and ceded most of his lands.

On 6 November 1813 Prussian troops bombarded the French occupied citadel and much of it caught fire, including the church and monastery buildings.

It could accommodate approximately 500 soldiers and was built primarily as an artillery position to hinder enemy access to the upper plateau of the citadel from the north.

In 1912–1913 it was remodelled and expanded with a neo-Baroque style mansard roof and the addition of large dormer windows on both sides of the building.

[21] After the end of World War I in 1918, as a result of the Treaty of Versailles in 1919, there was gradual decline in use of military facilities in Germany.

The barracks was used as residential building and as quarters for the civil police and the Freikorps Thüringen, a civilian militia army unit.

In 1944 a transit camp for displaced people was established in the Defense Barracks, and this was continued by the occupying Soviet administration after the war.

[22] Administrative offices of the Thuringian department for monument protection and archaeology (Thüringische Landesamt für Denkmalpflege und Archäologie, TLDA) are now housed in the building.