The old water tower hosts the multimedia water museum "Aquarius". The museum is one part of the "Route Industriekultur" - the Industrial Culture Route.
The building is very nice - from the outside and the inside. You don't only get a great view over Mülheim and its surrounding cities like Oberhausen and Duisburg from the top - you will also learn a lot about water and the environment.
Unfortunately the installations are only available in German so if you don't speak German don't bother going here!
the "ruhr" flows thru the city of muelheim. there are walking paths for taking a turn or cycling tours.
I visited here on the early spring, this city is really amazing, you just need to walk by the Ruhr river and enjoy the sites.
I only liked the look of this little castle - but I can't tell you anything about this place. Maybe some day...