In 1656 governor Arnold de Vlaming van Outshoorn brought the remote Ambonese island of Buru under control of the VOC. The following year governor Jacob Hustaert (1656–1662) signed a treaty with the chiefs of Buru to destroy all clove trees –to control the VOC monopoly
and to concentrate the island inhabitants in one village in the Bay of Kayeli. Here the VOC built a wooden barricade, called Mandersjah (named Sultan Mandarsyah of Ternate, the official ruler of Buru). In 1661, on request of the local population, Simon Cos replaced the temporary wooden construction by a stone redoubt and named it after himself. Orders from Batavia forced him however to change the name from Cosburg into Oostburg. In
1668 the fort was damaged and plundered during a revolt. When order was restored it was repaired. In 1689 during the visit of governor Dirk de Haas (1687-1691) the powder house accidentally exploded and the fort burned down. On the same spot a wooden fort was now built, called Defensie. Under governor Bernardus van Pleuren (1775-1785) the primitive wooden stronghold by a proper stone fort. In the period 1778-1787 four men overlooked the design and construction of the new fort: Lambertus Jansz. Haga, Wijnandus Johannes de Bourghelles de la Vacquerie, Jan Bruyns en Elias Mazel. Their names can still be seen chiseled in the fort. The fort consisted of a square with four with a half-bastion at each of the four corners. The fort remained in service well into the 19th century. It had a garrison of about 25 men under a lieutenant until it finally lost its permanent military function in 1867. The ramparts of the fort, complete with bastions, sentry boxes and gates, have survived to this day. The buidlings which once stood inside and surrounding the fort have disappeared.
