You don't have to finish, as far as I'm aware. Go downstairs, into the sewers, and you should reach a path that with two gates opposite each other. The diagram is in a chest behind the wall in between those gates, that I think you need to find a switch on the wall for.Get Junior
But I can not open that gate Unless there actually is some sewer entrance somewhere else and that is the 'exit spot' or something. If that is the case I feel very stupid
I can see the breakable wall if I go through to the
Junior mansion basement entrance
And unrelated. Holy shit. The music/atmosphere in Skellige is god tier. Amazing stuff so far.