Definition of fastness noun from the Oxford Advanced Learner's Dictionary

 

fastness

 noun
noun
BrE BrE//ˈfɑːstnəs//
 
; NAmE NAmE//ˈfæstnəs//
 
(literary)
 
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a place that is thought to be safe because it is difficult to get to or easy to defend synonym stronghold a mountain fastness The enemy retreated to their fastness in the mountains. Word Origin Old English fæstnes (see fast ‘quick’ and -ness).
See the Oxford Advanced American Dictionary entry: fastness