Actually the concept of "a good defense is the best offense" seems quite applicable to Civ, a couple of different ways:
-- since the game includes ways to win which are not military, one possible path is being impenetrable enough militarily to buy the time needed to carry out a cultural or diplomatic victory, or simply a time victory (highest game score). Of course not all games played include all those victory options, but if it does then they are a type of "offense."
-- even in games that don't include cultural/diplomatic/time victory options, the "cottage spam" approach is ultimately a type of "good defense becoming the best offense" military strategy. The idea is to spam cities/cottages and do certain other things which leave you quite vulnerable for a good while, but at some point abruptly provide you with the means to purchase a lot of the biggest/nastiest armaments and go crush the other civs. The hard part is surviving intact long enough to do this, and against experienced human players it really requires having decent defenses all along. (The costs of which in turn make carrying out the strategy a bit more difficult.) So this strategy is really, "a good defense buys you enough time to suddenly come roaring out with an overwhelming offense."
-- and of course the traditional version that applies to any decently-balanced strategy game is to lure opponents into exhausting themselves pounding without success against your defenses, and then counterattack. I lost a 3-way team game online like this just the other day: my teammates and I concluded that we should attempt to wipe out one of the other teams; we threw everything we had at them and did some damage but failed to kill them; having weathered the storm they came back at us with more/better units than we could hold off with our temporarily-depleted military. (And the third team was smart enough to see their opportunity, and ended up winning, the vultures!)
In single-player games, you have to go early war in Immortal and Deity.
