The oldest part of the present Bossenstein Castle is the square keep, probably built before the 14th century by a Joannes van Busco or Van den Bossche. Not much later it went to the Van Berchem family. They are supposed to have made some major alterations to the castle. They sold it in 1544 to Guilelmus van der Rijt, who was a member of the city council of nearby Antwerp. In the deed of sale the castle was described as an impressive estate, with a lot of outbuildings. Of these outbuildings nothing survives.
In 1655 it was sold again, this time to Willem van Halmale. He rebuilt the castle into a beautiful residence, which was still fortified. During the 18th century the castle changed hands several times between several noble families. Between 1798 and 1843 the castle was used by religious institutions. In 1906 the castle underwent restoration works, saving it from slow decay.
At present the castle is surrounded by the grounds of an 18-hole golf and polo club. The terrain of this club is probably not freely accessible. There is however a long distance walking path leading through the golf course which is. As far as I know the castle itself can not be visited.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.