The hospital was opened in 1868. By 1896 the hospital had grown to one of the largest in the country. The original hospital had 650 acres and about eighty buildings. When it first opened, it was automatically overcrowded, so they started to expand and expand. Most buildings are formed around the main building "Shrew Hall". Shrew Hall was one of the first buildings on site, and was named after the hospitals first superintendent. The site also included the Stanley Hall (maximum security building), Woodward Hall, Weeks Hall, Russell Hall, Beers Hall, Smith Home (employment housing), Shepard Home, and a Police station. The patients lived their days by bells, patients would know when to wake up, meals, exercise and bed times when a loud bell would ring to alert them. By 1890 the patient ratio was 230:1 meaning patients were not getting the adequate care they needed. When new superintendents took charge of the hospital they organized the patients more efficiently, making patients with the same symptoms live together. A treatment the hospital used heavily was hydrotherapy, where patients were forced into ice baths or really hot baths to "help" with their mental illness. The site also has a cemetery that holds 1,652 unmarked graves of patients. One of the most notable patients was serial killer Amy Archer-Gilligan, who worked at a nursing home and killed 48 patients by poisoning them. She was sent to Valley hospital in 1924 when she was declared insane, and remained until her death. The hospital is still used, though 24 buildings on the land are abandoned and will soon be demolished.
Because this is on site of a still active hospital and because there aren't many photos online of the inside, I am guessing they are pretty hard to get into, and you could easily be caught.