Where does “fortress” come from?

Fortress comes from Medieval Latin fortalitia, a derivative formation from fortis meaning strong.

fortress (English): A fortified place; a large and permanent...

Definitions

Step	Language	Word	Meaning
1	Old French	forteresce	fortress
2	Medieval Latin	fortalitia	—
3	Latin	fortis	strong, powerful; firm, resolute, steadfast,...
4	Old Latin	forctis	strong
5	Proto-Italic	forktis	strong, robust
6	Proto-Indo-European	bʰerǵʰ-	to rise; high, lofty; hill, mountain

fortress — Etymology, Origin & Word History | Etymology Explorer