Reinforced concrete and a solid footing seem to be the glue holding this derelict structure together despite so much of it hanging off on its various sides, including a walkway-to-nowhere suspended out behind it (on the land side).
More on the local military use history of the site via Wikipedia: “Devil’s Slide was the location of a military triangulation station and observation site used during World War II as part of the harbor defense of San Francisco. Prior to the advent of radar, military personnel would use binoculars and compasses to search for ships at sea and relay their coordinates to a central post.”
“By combining information from multiple observation posts, a ship’s precise location could be determined by triangulation. There were six military structures at the Devil’s Slide: three concrete and steel observation pill-boxes, two concrete and earth bunkers, and a reinforced steel observation tower. The pill-boxes were used as hardened observation posts, and one of the bunkers was used as a communications and command post. The site was sold to a private owner in 1983, but some of the structures remain.”