Talk to Jimmy and tell him about the "big gate". He'll give you a rope so use it on the collapsed wall. Hide behind the walls so the level 90 Dark knights can't see you. Go south till you reach a fountain then go west. Run all the way to the end until you come up to another fountain then go north. After you reach a path follow the path to a ladder and a guard will be guarding it. Kill him and go up the ladder. Go south to a small shed and take the hammer inside. Go back to the ladder and use the hammer on the nearest window. You'll then break the window so click the broken window to enter the fortress.