Go up to where the estate is located, past the 2 wolves, and head right, there should be stairs. Go down and head straight then left, there are sewers, go straight then turn right, keep going straight and avoid the enemies, unless you want to fight them, up to you. You should see a ladder at the end. Once you have gone down the ladder, go off the platform, then head right and straight, there should be an entryway into some tunnels, there is where you get the sewer key for the area where Gascoigne is.
