My absolute favorite part of my visit to Alcatraz last August was exploring the area beneath Building 64 — the area known as “China Alley.” The upper (concrete masonry) portion of Building 64 wasn’t built until the early 1900s, some 40 years after construction of the arched brick foundations. Those upper levels were built to house the military and prison staff working on the island. But because of structural damage and deterioration over the years, the residential quarters are closed to the public. You can get a few glimpses of the rear of the structure as you stand below, in China Alley.
There is so much architectural, social, military, and penal history in this one building alone, stretching from the Civil War era of the 1860s all the way through to the Indian occupation (which ended in June 1971) and the National Park Service’s efforts to restore and preserve the site. If you have some time to spare and love looking at historical photographs, you might enjoy browsing through a PDF, published in 2003 by the National Park Service, titled Alcatraz Barracks Building 64: Abbreviated Historic Structure Report.