Harderwykenburg is one of the oldest extant castles of East Frisia. It was built in the style of a medieval stone house soon after 1450 in Leer.
The original building, in the form of a tower, was built of stones from a monastery, and measures 8.09 x 11.13 meters. The external walls are between 0.96 and 1.23 meters thick. The tower was later supplemented by a two-storey extension. The steeply proportioned building has now been not entirely satisfactorily restored.
The first owner of the castle was probably the chieftain Hayo Unken, on his mother's side a grandchild of the chieftain Focko Ukena. The castle was also known as the 'Unkenburg', after Hayo's family name. In 1588 the steward (Drost) Dietrich Harderwyk Armgard married a daughter of Hayo Unken IV, and thus came into possession of the castle, which now bears his family name.
In 1788 possession of the building passed from the then owner, Carl Stephan von Schilling, to Carl Gustav, Baron zu Inn und Knyphausen, whose descendants continue to live there.
References:Doune Castle was originally built in the thirteenth century, then probably damaged in the Scottish Wars of Independence, before being rebuilt in its present form in the late 14th century by Robert Stewart, Duke of Albany (c. 1340–1420), the son of King Robert II of Scots, and Regent of Scotland from 1388 until his death. Duke Robert"s stronghold has survived relatively unchanged and complete, and the whole castle was traditionally thought of as the result of a single period of construction at this time. The castle passed to the crown in 1425, when Albany"s son was executed, and was used as a royal hunting lodge and dower house.
In the later 16th century, Doune became the property of the Earls of Moray. The castle saw military action during the Wars of the Three Kingdoms and Glencairn"s rising in the mid-17th century, and during the Jacobite risings of the late 17th century and 18th century.