Defending Australia  Building sandcastles, we fair gloried in their ephemeral lives. Moat brimming with cool ocean swell, clear water circling a Keep dumped from upturned bucket. Loopholes for archers, paddlepop sticks for a palisade, shells as shields, slimy seaweed strips lacing speckled ramparts.   But we always knew collapse loomed, that our fortress faced a reckoning: from sudden rush of surging breaker, its blanketing cascade razing our battlement; from louts with big-toed battering rams; from slice of surfboards planing on shoreline; from lunar ebb and flow.   Behind us hulked real forts; total war’s bunkers with slit cut lookouts like…