American English
/ˈfɔɹ.tɹəs/
2 syllables · Stress the first syllable.
A large, strong building or group of buildings with thick walls, used to defend a place.
Practice out loudA large, strong building or group of buildings with thick walls, used to defend a place.
The army defended the fortress for three weeks.
Tourists visited the old fortress on the hill.
medieval fortress · fortress walls · defend a fortress · capture a fortress
a fortress with [thick walls]
defend/capture a fortress
A person, group, or place that is very difficult to attack, enter, or change.
The company was a fortress against outside competition.
Her home became a fortress of calm during the crisis.
This meaning is often figurative: it describes something that is difficult to enter, attack, or change.
an impenetrable fortress · a fortress against change
a fortress of [something]
remain a fortress against [something]
The plural is fortresses, with -es.