[syn: stronghold, fastness]
The Collaborative International Dictionary of English v.0.48:
Fastness \Fast"ness\, n. [AS. f[ae]stnes, fr. f[ae]st fast. See
   Fast, a.]
   1. The state of being fast and firm; firmness; fixedness;
      security; faithfulness.
      [1913 Webster]
            All . . . places of fastness [are] laid open. --Sir
                                                  J. Davies.
      [1913 Webster]
   2. A fast place; a stronghold; a fortress or fort; a secure
      retreat; a castle; as, the enemy retired to their
      fastnesses in the mountains.
      [1913 Webster]
   3. Conciseness of style. [Obs.] --Ascham.
      [1913 Webster]
   4. The state of being fast or swift.
      [1913 Webster]
WordNet (r) 3.0 (2006):
fastness
    n 1: a rate (usually rapid) at which something happens; "the
         project advanced with gratifying speed" [syn: speed,
         swiftness, fastness]
    2: the quality of being fixed in place as by some firm
       attachment [syn: fastness, fixedness, fixity,
       fixture, secureness] [ant: looseness]
    3: a strongly fortified defensive structure [syn: stronghold,
       fastness]