Civilization – In Civilization IV what are the differences between the Difficulty Levels

From my experience, 3 military is good enough for average defense, it allows you to hold out till you can reinforce.

But for your style of play, perhaps 5-10 would be better, you don't want to many as it causes slow down in tech etc..., but then you don't want to get crushed. You should have more on the outer cities and reinforce from your inner ones, I try to always have 3 minimum in all cities, but doesn't always work, then any bigger cities up it to 4-5.

If you notice your enemies normally attack you with massive stacks, the only way to counter those are with a large stack yourself. Have 1 large stack that walks between cities and does the attacking, then leave 3 units in all your cities for normal defense, remember if you suddenly get attacked at a choke point (hopefully you have one) you can always reinforce from your other cities, knocking them down to 1-2 stuff like that.

If you go to the Victory Conditions screen (I believe it is F8) it should show you what is required for a cultural victory based on your game length. The left hand column shows how close you are to each condition, but the right hand column should show which of your opponents is closest. So you can watch this to see if they are gettting close to a cultural victory, and if so, attack the cities as mentioned in the other answers.