One comment to mauld- if it should happen that a hotel ask you to pay for a drink/coffee/etc after having offered it- reference to the bartender/server/manager that you were invited as your room was not ready as promised. Typically, they don't know/don't remember- and will quickly charge it to the appropriate rooms division account for this purpose.
This has happened to me at a ton of hotels- SPG, MO, LHW, even Fs and ShangriLa- so just be a bit firm about it and it should work out- provided you don't order a bottle of the aforementioned champagne
Koln is a beautiful city- and the Dom is a nice hotel- give it another chance!