The Valkhof is on a hill overlooking the river. It is the site of a former Charlemagne fortification and the surviving Carolingian elements are quite modest. There are two buildings with a Carolingian element. The first is an octogon chapel built in the style of Aachen in the 8th or 9th century. The initial building was constructed about 1000 and rebuilt about 1400. It used material from Charlemagne's fortification and we think we could identify some of these Caroilingian flat bricks. The second building is the ruin of Barbarossa's chapel that incorporated several Carolingian capitals on Roman pilars in the chapel.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.