From Latin 'fortis' (strong) through Old French — sharing its root with 'fort,' 'fortify,' 'force,' and 'comfort.'
A large, strong building or group of buildings designed to resist attack; a heavily fortified place.
From Old French 'forteresse' (a strong place, a fortress), from Vulgar Latin *fortalitia, from Latin 'fortis' (strong, brave, robust). The Latin adjective 'fortis' descends from Proto-Indo-European *bʰerǵʰ- (high, elevated) or possibly *dʰerǵʰ- (to hold firmly). The same root produced 'fort,' 'fortify,' 'comfort' (to strengthen greatly), 'effort' (a putting forth of strength), 'force,' and 'forte' (one's strong point). A fortress is, at its core, simply 'a strong thing
The word 'comfort' is a secret sibling of 'fortress.' Both come from Latin 'fortis' (strong). 'Comfort' entered English from Old French 'conforter,' from Late Latin 'confortāre' — literally 'to strengthen greatly' (con- = intensive + fortis = strong). When you comfort someone, you are etymologically making them stronger, not softer. And 'effort' is from Old French 'esfort' — a putting