The remains of the Fortress can be visited
FortressDüsseldorf-Altstadt
Fortress Düsseldorf

The city of Düsseldorf was already surrounded by a wall and moat when it was founded in 1288. In the 14th century, a stone wall with towers and gatehouses was added. With the advent of firearms in the late Middle Ages, the medieval wall was initially reinforced with roundels around 1500.
Source: duesseldorf.de · Image: http://www.wehrbauten.de/thread.php?threadid=572&sid=fabdb0784445e173f7d2c746486aca16
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Location: 51.2244, 6.7704