At least one person must have Farmer Kao's Shovel and be flagged to pick up the Drudge Fort Blueprints. Flagging requires bringing food to Sergeant Shanks. The blueprints and shovel are required to get the Cave Door Key. Bring the Blueprints to Hoogan the Banderling, barricaded inside the small hut. He will tell you that he requires a shovel to escape. He gives the key for the shovel.