fortress

Fortress entered English in the early 14th century from Old French forteresce/forteresse, a variant of fortelesse, which traces to Medieval Latin fortalitia — Latin fortis ("strong") plus the suffix -itia, used to turn adjectives into nouns of quality or condition.

The Old French form shifted its medial -l- to -r-, a change also seen in words like orne (from ulmus) and chartre (from cartula), giving us the shape we recognize today.

The same Latin -itia ending, filtered through French -esse, shows up in duress, prowess, largesse, and richesse — words that, like fortress, were originally built to name a quality rather than a place.