I was in Hamburg for less than 4 hours, between trains, but I love the city... there was a mediaeval fair called BritFest in the main square in front of the Rathaus (the central government building).
This port city has always enjoyed a sort of independent status, a city-state, free to do what it pleased - maybe that's why it's known for prostitution and sleaze!
Hamburg bears the title "Freie und Hansestadt" ("Free and Hanseatic City"), from its independent roots.
Heavily bombed by the allies during World War II (it was right on the way for bombing runs to other parts of Germany), it suffered the first known 'firestorm' of July 28, 1943, when high winds turned incendiary bombs into much more devastating killers than planned. 42,000 died and 21 square kilometres was razed.
As a result of its location during the war, not a great deal of the old city remains, but some gems can be found.
While I kept to the interior, not wandering too far away from the trains, Hamburg is in fact a spreading town, with lots of green spaces, canals, and it claims to have more bridges than even Venice!