You start this level on a dock outside the Order's castle in Hanover. Kill Black Raiders along the way, kill two guards inside, and shoot the button on the other side of the door. Door will open.
Front gate is closed and locked.
Jump down into the water, and follow the tunnel. Crawl under and follow around. Pick up a med kit, and crawl through the whole in the corner. You'll come out behind the locked gate that you couldn't get through. Be careful of rocket-armed raiders.