This Parade Ground?  https://www.nps.gov/places/000/alcatraz-parade-ground.htm

Sorry, but which branch of the military is using this decaying concrete surface in a National Park that's described as "...an excellent spot for a selfie...", "...host[ing] annual sunrise ceremonies which celebrate Native American culture..." and "...the trailhead for the Alcatraz Agave Trail and is open when the birds are not nesting..."?