The significance of Mr. Douglas brand and the moat lies in the cover that Detective Birdy Edwards needs to protect his identity from being discovered by the Scowrers, a murderous secret society that was busted by Edwards in the US. The moat offers physical protection to the house where Birdy Edwards lives. So, both Mr. Douglas brand and the moat are shields from the relentless enemies who were after his life after they were released from prison.
Explanation:
The "Valley of Fear" was the fourth and final Sherlock Holmes novel by Sir Arthur Conan Doyle. It was written about 1914. It chronicled the events that led to the discovery of Birdy Edwards, a Pinkerton detective in Chicago, who ran to England to escape from the murderous hands of a notorious gang. He was not aware that the society had members spread over the globe, including Professor Moriarty. The secret society never forgives and the members never forget. That is why they traced Edwards to England, though he had earlier disguised his identity by answering Mr. John Douglas and carved out a residence surrounded by a moat for physical protection.
