The repetition of the word, coupled with the image of a city under seige, calls attention to the slight pun here. This is indeed a "fort"-night, being a period of time in which human relations are defined by seiges and defenses--the realities of brute power.

A fortnight is equal to 14 days, which is roughly the amount of time that elapses between the riot and the arrival of Barrios' troops (though that interval is impossible to establish exactly due to the tangled chronology). Though Mitchell is referring to the cable and the steam service, his "fort"-night corresponds to the novel's night of Separatism.