Battery Mendel, named for US Army engineer George H. Mendel, is part of a complex of old coastal fortifications, called Fort Barry, on the north side of the Golden Gate. They were the counterpart of those at the Presidio on the south side: www.ipernity.com/doc/289859/49792646
Battery Mendel was constructed in 1905 and, altho rapidly outclassed by newer batteries, nonetheless remained in service till 1943, whereupon the cannons were sold as scrap, presumably for the war effort. Its cutting-edge innovation for the time was so-called "disappearing guns," which would fold below the parapet after firing so as to present less of a target. The insets show some different views, as well as an explanatory sign with more information. Just a note, too--the sky isn't overexposed: that's fog!
All these fortifications, like those at the Presidio, were thoroughly obsolete by the mid-twentieth century, and when the Army decomissioned Fort Barry in the 1960s it was only by a series of happy accidents that the property didn't end up developed commercially. As it is, the area is now all part of the Golden Gate National Recreation Area, with the old military installations managed as historic sites. The area also includes a Nike missile-defense site dating from the 1950s thru early 60s, which I'll post on one of these days--