From Longman Dictionary of Contemporary Englishfastnessfast‧ness /ˈfɑːstnəs $ ˈfæst-/ noun [countable] literary SAFEa place that is safe because it is difficult to reach SYN stronghold mountain fastnesses
Examples from the Corpus
fastness• And whom, casually, Thorfinn had now brought south from his fastness to greet his uncle.• It has been in its fastnesses for many years, each population evolving to its own rules.• In our North London fastness we are as ignorant as snowflakes of such matters.• It was appropriate that he should end his days in the masculine fastness of an Elizabethan almshouse in London.• Up there, amid mountain fastnesses, its waters are sweet.• But the great condor, sulking on some remote ledge in the fastnesses of its preserve, fails to appear.• Green and red and gold began to appear in the winter fastness around us. 