The correct answer to this open question is the following.
The significance of Mr.Douglas’s brand and the moat is that these represent a somehow kind of protection against the enemies of inspector Edwards. He needs something to defend against enemies and he uses both as a protective shield from the secret society he discovered.
This is part of the story "Valley of Fear," written by Arthur Conan Doyle, about the famous character Sherlock Holmes. Indeed, it was the final novel about the acclaimed detective Holmes.