Fastness, n.
Etymology: [AS. fæstnes, fr. fæst 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.
Shades of meaning
Fernald has no entry of its own for fastness; he lists it under fortification.
Where he writes about it:
Any defensible place, whether made so by nature or by art, is a fastness or stronghold. — from fortification
Antonyms
none given