Blankenhagen Castle was a lowland castle , whose ruins are located by the River Aller near Grethem in Lower Saxony, Germany. The motte-and-bailey castle is believed to have been built around 1200. It is supposed that there used to be fortified buildings on the two low mounds or mottes, and that a bailey was constructed on an outer island-like area.
The castle stood in the meadow landscape of the Aller valley about half a kilometre northeast of Grethem. It lies today between the Old Leine and Aller rivers. It is accessible via a track that leads to the ferry crossing over the Aller to Eickeloh.
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