And brought down, from their fortresses, those people of the Book (The Jews of Bani Qurayzah) who supported the invaders, and cast terror into their hearts. As a result, some of them you slew and some you took as prisoners.
Meaning:-
Fortress: any place of exceptional security; stronghold.
Invader: a person or group that invades a country, region, or other place.
Slew: to kill by violence. to destroy; extinguish.