Source : Webster's Revised Unabridged Dictionary (1913)

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.

            All . . . places of fastness [are] laid open. --Sir
                                                  J. Davies.

   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.

   3. Conciseness of style. [Obs.] --Ascham.

   4. The state of being fast or swift.

Source : WordNet®

fastness
     n 1: a rate (usually rapid) at which something happens; "the
          project advanced with gratifying speed" [syn: {speed}, {swiftness}]
     2: the quality of being fixed in place [syn: {fixedness}, {fixity},
         {fixture}, {secureness}]
     3: a strongly fortified defensive structure [syn: {stronghold}]