Bay senior history buffs who feel cheated because we have no blood-stained battlefield sites here will have to make do with the military fortifications of the Marin Headlands. This Sunday from noon-4 you can take a docent-led tour of the recently restored Battery Townsley. Built in 1940 this once secret site waited for an attack from the Japanese that never came. Just in case, its massive two-ton battleship guns could hit targets 25 miles out to sea. The labyrinth of tunnels underneath the fortification have been fully restored thanks to the estimable Golden Gate National Parks Conservancy. Park at Rodeo Beach and hike up the Coastal Trail for half a mile.