In Hamm there was a redlight district establishment with maybe 4 or 5 occupied windows which has closed back in 2011.
But the local newspaper reports that girls can still be found standing outside. There are no 'Sperrbezirke' in Hamm so this still legal.
Directions: Gallberger Weg, south of the town centre, off Alleestrasse or Richard-Wagner-Strasse. By walking, you could follow the railway line southwards (eventually you follow an unfenced section of the railway) and turn off on Klutestrasse.
Gallberger Weg does meet the railway further south, although it is only a walking path at this point. The zoo nearby is a good landmark.
You can read more from: http://www.wikisexguide.com/wiki/Germany