Archaic KJV Word
Fortress
Modern equivalent: safe place
What Was Lost
The military desperation. David wrote 'God is my fortress' while hiding from armies in actual mountain strongholds. A fortress was not a nice building but a last-resort defensive position -- high on a cliff, walled, with limited access, where a small force could hold off an army. 'God is my fortress' meant 'God is my last-stand defense when everything else has fallen.'
Closest Survivor in Modern English
fortress (still used but the military-survival urgency has faded to poetic imagery)
Peak Usage (1611)
KJV Psalm 18:2 -- 'The Lord is my rock, and my fortress, and my deliverer'; Psalm 91:2 -- 'My refuge and my fortress'
Died still used but romanticized (~1800)
Hebrew metsudah ('mountain stronghold/fortified high place/virtually impregnable defensive position') romanticized into fairy-tale castle imagery. The desperate, military, survival dimension was lost to tourism.
What Replaced It
“castle”
Romantic and scenic; a fortress was a last-stand defensive position when your life depended on its walls
“stronghold”
Closer but abstracted; metsudah was a specific geographical reality -- high, walled, and defensible
“safe place”
Casual; a fortress was where you went when the alternative was death