Fortress

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Middle English: from Old French forteresse ‘strong place’, based on Latin fortis ‘strong’.

wiktionary

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Early 14 c., from Old French forteresce, forteresse, forterece(“strong place, fortification”)[from 12th c.], variant of fortelesse, from Medieval Latin fortalitia, from Latin fortis(“strong”) (see fort) +‎ -itia, added to adjectives to form nouns of quality or condition. French -ess, from Latin -itia is also in words such as duress, largesse and riches. For change of medial -l- to -r- in Old French, compare orne(“elm”) from ulmus; chartre from cartula and chapitre from capitulum.

etymonline

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fortress (n.)

early 14c., from Old French forteresse, forterece "strong place, fortification" (12c.), variant of fortelesse, from Medieval Latin fortalitia, from Latin fortis "strong" (see fort) + -itia, added to adjectives to form nouns of quality or condition. French -ess from Latin -itia also is in duress, largesse, riches, also obsolete rudesse, "lack of cultivation" (early 15c.).

For change of medial -l- to -r- in Old French, compare orme "elm" from Latin ulmus; chartre from cartula; chapitre from capitulum.