The remains of the Castle can be visited
Sibetsburg Castle

The remains of Sibetsburg Castle, once a pirate stronghold, are now located in the Siebethsburg district of Wilhelmshaven. Originally situated on an arm of the sea that allowed direct access for ships, the site had a past on the North Sea coast.
History+
The castle was built in 1383 by Chieftain Edo Wiemken under the name Edenburg. His grandson, Sibet Lubbenson, renamed it Sibetsburg in 1416. For a long time, it served as a retreat for the Victual Brothers and was protected by several ramparts and moats.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://de.wikipedia.org/wiki/Datei:Sibetsburg,_oben.jpg
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 53.5317, 8.1077