original: they were built mainly by nations like germany during world war ii to protect important industries from aerial bombardment .
simplifications: they were built mainly by countries like germany during world war ii to protect important industries from attack by planes . they were made by countries like germany during world war ii to protect buildings from planes that dropped bombs . they were built in germany during world war ii to protect important industries from aerial bombardment . they were built mainly by nations like germany during world war ii to protect important industries from bombing from the sky . they were built mainly by nations like germany during world war ii to protect important industries from air bombardment . they were builty mostly by nations like germany during world war ii to protect important industries from air attacks . nations like german built them during the world war ii mainly to protect important industries from aerial bombardment . they were built mainly by nations like germany during world war ii to protect important industries from aerial bombardment .