Because the island on which the castle was erected is separated from the shore by at least two bodies of water, artificial defences such as moats or shield walls were usually unnecessary if the castle was surrounded by flowing water. Such castles could therefore be very easily and cheaply built. Many island castles in lakes were, however, relatively easily captured in winter if there was an ice sheet thick enough to support attacking troops, because they were often rather poorly fortified.
- Turnbull, Stephen (2003). Crusader Castles of the Teutonic Knights (1): The Red-brick Castles of Prussia (1230-1466), Osprey, Oxford. ISBN 1-84176-557-0.
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