Witcher 3 a gifted tome?

I did not help the guy with the vampire back then. Now I can't find an entrance anymore. Can anybody help me further. Play it on the Xbox

Go from the rosemary to the north at the small market place (there's a quest with a food thief) over to the local exit gate. At the side of the gate, you will find an entrance to the wall high or wall tower, so actually for the guards. Climb down into the sewage vault.