Battery Townsley - Marin Headlands
A World War II battery (and later, an underground testing facility), Battery Townsley was restored and then reopened to the public in October of 2007. It had languished in a bad state for a number of years before a bequest allowed for the fort's restoration.
The National Park Service website has photographs of the 16-inch guns that once inhabited Battery Townsley.
Read about Battery Townsley's ribbon cutting ceremony in this October 2007 piece from the San Francisco Chronicle.
The battery is open to the public on the second Sunday of each month. You can, however, walk through parts of the structure for a cursory view. See the NPS website for details.