From what I can find on the internet, this was a platform for a radar that would have controlled Nike Missiles during the Cold War. The location is marked as Hill 88 on the park map. The park has a bit more info on the Nike missile site but doesn't really talk about the radar site on Hill 88. There was another Nike/radar site on the top of Angel Island and only within the last few years have they restored the top of the island (for the military installation they flattened the top of the island). The story of how the headlands came to be preserved as a parkland is an interesting combination of wisdom and luck.