HH Holmes is one of the first documented American serial killers. One of his sickest and notorious schemes is a “Castle” he built in Chicago that he opened at the time of the 1893 World’s Fair as a hotel. The hotel had numerous rooms and ways to kill people and it was so effective, that it’s unknown how many victims he claimed.
Holmes bought a lot in the Englewood neighborhood of Chicago and started building. The “Castle” as it was dubbed by the neighbors, was a full block in area and three stories high. He constantly changed the builders so that he was the only one who knew the full layout of the place. It has a maze of over 100 windowless rooms with doorways opening to brick walls, oddly-angled hallways, stairs to nowhere, door that only opened from the outside… overall a really creepy place.
After the hotel was completed, Holmes selected mostly female victims from among his employees, and guests that came in from the world’s fair. He would torture and kill them by doing things like locking them in soundproof bedrooms with gas lines that let them asphyxiate.
The entire story is fascinating. You should read more about it here.