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CONTAINS SPOILERS
Secret behind a powerful Witch's house
Jefferson's clue "Behind a powerful Witch's house lies a secret, accessible from the west side... To find it you must look for openings in the hedge..." refers to a small opening in the west hedges right outside of the Ceramic Manor (see image). Go west and walk into the bushes next to the opening. Walk into the pool to find the next opening which leads to the secret behind the house.