The booking of your overnight stays is on your own authority. The Köln Tourismus GmbH supports you in finding a room. 

Of course it's possible to book a room directly in the venue - the Maritim Hotel of Cologne:

MARITIM Hotel of Cologne
Heumarkt 20
50667 Koeln (Cologne) / Germany
Tel.: +49 (0) 221-2027-0
Fax: +49 (0) 221-2027-826