Hm, no I didn't even notise the sewer entrance.
I hate to sound like an idiot, but I must be missing a lot of things in this game. How do I free Mordaneus from being locked in his own house and how do I enter the dwarven mines? I talked to the dwarven king and he marked the entrance on my map but when I go there I can't find anything on how to get into the minds...