The Arenberg site had been the castle of the lords of Heverlee since the 12th century, but this family became impoverished and had to sell the site in 1445 to the Croÿ family from Picardy. Antoon van Croy demolished the medieval castle and started works to build the current château in 1455 on the site, of which he destroyed all but one tower. Willem van Croÿ completed the works on the château in 1515, and founded a monastery on the château grounds for the Benedictine Celestines. The architectural style is in large part traditionally Flemish, with sandstone window frames and brick walls, though it has been structurally altered since 1515 and has elements of Gothic, Renaissance and Neo Gothic architecture. Its large corner towers are typical, once surmounted by a German eagle.
Charles III of Croy was the 4th and last duke, and after his death in 1612 without issue the château passed to the Arenberg family into which his sister had married, and remained in that family until the First World War.
During the First World War, the château and grounds were occupied by the Germans and Austrians. The château and park were seized by the Belgian government on the outbreak of, and then after the war since the Arenberg family was considered to be German or Austrian due to their close Habsburg connection, monarchs of Austria-Hungary. It took until 1921 for the University to acquire them, becoming an expanded natural sciences and engineering campus in the style of that of an American university. After the partitioning of the university along language lines in 1968, the château and grounds remained with the Dutch speaking half as one of the main campuses for the new, independent Katholieke Universiteit Leuven. The château itself is the main building of the Faculty of Engineering and houses lecture rooms and studios for the Department of Architecture, Urbanism and Urban Planning, including the Post-Graduate Centre Human Settlements and the Raymond Lemaire International Centre for Conservation. The building is open to the public. The former Celestine monastery on the château grounds now houses the campus library, and the addresses of many of the science buildings are on the street named Celestijnenlaan (Dutch for 'Celestine Street').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.