“Holmes [opened his hotel] for the World’s Columbian Exposition in 1893 [...] The ground floor of the Castle contained, aside from Holmes’ own relocated drugstore, various shops [...] while the upper two floors contained his personal office as well as a maze of over one hundred windowless rooms with doorways that would open to brick walls, oddly angled hallways, stairways to nowhere, doors that could only be opened from the outside, and a host of other strange and labyrinthine constructions. [...] Over a period of three years, Holmes selected female victims from among his employees [...], lovers and hotel guests, and would torture and kill them. Some were locked in soundproof bedrooms fitted with gas lines that permitted him to asphyxiate them at any time. Some victims were locked in a huge bank vault near his office; he sat and listened as they screamed, panicked and eventually suffocated. The victims’ bodies went by a secret chute to the basement, where some were meticulously dissected, stripped of flesh, crafted into skeleton models, and then sold to medical schools.”
There’s a book on this.