This is the museum to visit for everything you need to know about what went on in Cologne leading up to, during and directly following WWII. There is a very detailed exhibition, a great audio-guide and you can also opt for private tours. For those wanting to delve a little deeper, there's also an extensive library.
Most German cities offer a lot of information on WWII, however, due to Cologne's extremely rich Roman history, there is more of a focus on that part of the history. If Cologne is your only stop in Germany (or even if it isn't), a visit to this museum will show you exactly want went on in the city and around the country during the WWII years.
The museum itself is a ex-Gestapo prison which has cells that have been preserved.
Tu-Fr: 10:00-18:00
Sa-Su: 11:00-18:00