The remains of the Castle can be visited
Hohensyburg Castle

On the Syberg in Dortmund is the Hohensyburg castle ruin, with views over the confluence of the Ruhr and Lenne rivers into the Hengsteysee.
History+
The earliest settlement in the Syburg area can be traced back to the Stone Age, with initial traces of settlement from the Bronze Age around 700 BC. The first documented mention of Hohensyburg, then known as the Saxon Sigiburg, occurred in 775, when Charlemagne conquered the fortress and used it for defense against the Saxons. The following year, the Saxons besieged the Frankish castle but were surprised by a sally of the garrison and pursued to the Lippe river. Remnants of three castles were located on the mountain. The construction of the stone Hohensyburg is dated to around 1150. It served as a hilltop castle for the Archbishops of Cologne and was part of the territorial expansion in the 12th century. The Lords of Syberg were first mentioned in 1235 as ministerial officials of Cologne.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://www.wehrbauten.de/thread.php?threadid=553
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: 51.4200, 7.4869