skyline. Until the end of the 13th century, Emperors of the Holy Roman Empire and German kings often held court in the church. Church synods also took place there. 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. During the Thirty Years' War (1618–1648) Erfurt was occupied from 1631 by the troops of Gustav II Adolf of Sweden. In 1632, the Swedes briefly dissolved the monastery and temporarily converted it into a Protestant monastery in 1633. After the occupation only three monks remained. The church was damaged and the treasures and stores of the monastery were plundered. The damage to the church was repaired in the first and second building phases of citadel, in 1672 and 1727. In 1765 its interior was baroqueized by Italian plasterers. In 1803 the monastery was secularised. On 6 November 1813 Prussian troops bombarded the French occupied citadel and much of it caught fire, including the church and monastery buildings. The monastery buildings, other than the church, were later then completely removed by Prussian forces who took possession of citadel in May 1814. From 1820 the remaining church was used as a store for flour and military provisions. The towers were dismantled, a new roof was built and three storage levels were constructed inside. In spite of these alterations, the building still has traces of medieval and baroque architectural details, sculpture and paintwork, and the monumental size of the building can still be appreciated. Since 1993 the church has been used as an exhibition space for international concrete art, i.e. art which places a strong emphasis on geometrical abstraction. It has nothing to do with concrete, the building material. Defense Barracks (Defensionkaserne) The Defense Barracks, by far the largest building on Petersberg Citadel, was built from 1828 to 1831 by the Prussian administration, on the site of the former monastery building which had been damage by fire in 1813 and later demolished. The building is 167 metres long and 18.8 metres wide. The external walls are up to 2 metres thick. 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