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1. A fortified structure used to furnish protection against enemy attack in frontier areas, usually at a location of strategic importance; often square or polygonal in plan; typically constructed of hewn timbers having dovetailed notches at the corners to provide strong rigid joints; commonly, an overhanging upper story; often masonry walls on the ground story with log construction above, or entirely of log construction; frequently, a pyramidal roof; usually a few small windows with heavy shutters; loophole openings through the walls permit the firing of guns over a wide range of angles.
2. A reinforced concrete structure that provides shelter against the hazards of heat, blast, or nuclear radiation.