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:Bomarsund fortress and garrison was built in 1832-1854 by Russian Empire to defence Åland Islands against enemies. British and French fleet attacked against it during the Crimean War in 1854. After a week of fighting the British stormed the remaining fort. Anglo-French forces destroyed it totally after battle. In the Treaty of Paris 1856, the entire Åland Islands were demilitarized, which is a status that has been preserved until this day.
Nowadays there are some ruins left and fortress area is open for visitors.