Fastnes or Fastness: How To spell?
The word Fastnes is misspelled against Fastness, an adjective meaning "A fast place; a stronghold; a fortress or fort; a secure retreat; a castle; as, the enemy retired to their fastnesses in the mountains."
Fastness has double 'ss' in the end mispronounced as 's'Remember double 'SS' appears in the end of this word which should not be misspelled with single 'S'
Wrong Spelling:  fastne-s  Correct Spelling:  fastne-ss 
Explore spell check of Fastness to spell without errors.