Angel Island housed the Immigration Station and the Quarentine Station for those immigrating to the US through the west coast. The clearing and structure facing us is a Civil War era military camp. An explanation from the Angel Island web site:

"With concern mounting over threats to the Bay Area from Confederate sympathizers and naval forces, the federal government established Camp Reynolds (also known as the West Garrison when the whole island was Fort McDowell) on Angel Island in 1863."