Already mentioned by AlcatrazUK, but nevertheless...
Played quite a lot of civ, but due to my nature I always try to play competitively. But, due to that, I really missed all the achievement staff by /=
So, find a multiplayer game(~ +2000 hours guaranteed) and after go for real "achievements", like playing FFA and calling the first guy you meet a "f*uckface" in a general chat and winning the game after that
Besides that, you can almost always find simply better players or players who have some serious discrepancies with your strategical model. And it's lots of fun to find out which conditions define who's right and who's wrong.
Being rushed by a skilled player who backstabbed you after "may i plz dow you, want to pass through", having your last city with 1 hp and asking random players in game "please 61 gold for archer, imma dying and you'll be next if he takes me" when it's about 10 seconds left at timer, after which doing 3 moves faster than he could do two and seeing "WTH where did that archer come from?" and ultimately coming back into the game, being tightly allianced with the initial agressor and fighting civs like an era ahead toe-o-toe is quite an experience.
And suddenly you understand why +30% city ranged strength pantheon is in game;
and suddenly you rush Oligarchy as you saving grace, while normally you delay taking it for as much as possible.
And the game is interesting again
