Hi stales,

Nothing to do with forts, I'm afraid. It's a contraction of "fourteen nights". Merriam-Webster's online dictionary gives this:

Main Entry: fort·night
Pronunciation: 'fOrt-"nIt, 'fort-
Function: noun
Etymology: Middle English fourtenight, alteration of fourtene night, from Old English fEowertyne niht fourteen nights
Date: before 12th century
: a period of 14 days : two weeks

At least one other dictionary has it as "British and Australian". I know from previous discussions on the board that our Kiwi mates also use it. Try using the Search function for the word 'fortnight' (ignoring the thread "Challenge of the Fortnight"). You might like to also search for the much rarer 'sennight' (= seven days, a week) which old (cool) hand tsuwm cast before a stunned readership way back when.