Moriarty, or the man claiming to be him, describes himself as a "spider in the center of a web" in the episode. This is based on a quote in The Final Problem by Sherlock Holmes describing Moriarty.[1]
Sherlock calls Irene Adler "The Woman" in this episode, and that in his eyes she eclipses and predominates the whole of her sex. A Scandal in Bohemia opens with "To Sherlock Holmes she is always the woman. I have seldom heard him mention her under any other name. In his eyes she eclipses and predominates the whole of her sex."[2]
The house at the end of the episode where Irene Adler is found, is a real and is called the Commandant's House on Evans Street in Brooklyn, New York.
