I was there myself -October 2018- and the guided tour was the only option. You can inly walk some steps among the buildings inside the property and visit the small exposition in the reception/ticket office. That is also worth it if you cannot stay but you want to get a glimpse of the man and mission. You miss the inside thiugh.
It appears it only has guided tours as that is the only option mentioned on the website. http://www.parks.ca.gov/?page_id=489
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