Alcatraz is a large rock in San Francisco Bay, originally barren and covered with white pelican droppings. It is about a half mile long, 525 feet wide and 140 feet high.

After being acquired by the United States in 1848, it was turned into a fortress beginning in 1853. Throughout the 19th Century its buildings were mostly of masonry and brick (the fortifications, citadel, sally port, etc.) or wood frame (living quarters, warehouses, and other support structures).

As the island's importance as a prison eclipsed its importance as a fortification, a complex of newer prison and support buildings was erected during the early part of the 20th Century. During this time the old fortifications and their support buildings were obliterated, partially obliterated, or altered, and replaced with buildings of steel-reinforced concrete.