Civilization IV: How to tell what version I have

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.

Yes, increasing the global culture rating will increase the culture produced by each city, which will increase the rate at which your borders grow. With the right buildings, the global culture rating also adds happy people to your cities, which helps them to grow larger.

If you hover over a square, you can see the current balance of culture in that square just above the unit picture. Whoever has the highest total is the current owner of the tile. If your culture that reaches that square is higher than the culture of the other civ, your percentage will grow over time.

In your own cities you can also see a percentage of culture. If you captured the city from someone else, it will start out heavily waited towards them. The higher the percentage is for opposing countries, the more likely a city is to defect. You can't see a percentage of culture in cities you don't own, but it works similarly. If you have significantly higher culture output near an opponent's city than they do, you can eventually get that city to defect to you without warfare.

You need to be careful of capturing a city that is well surrounded by an opponents cultural borders, as it will likely defect back quite quickly. In this situation it is often better to just raze the city instead. You can decrease the likelihood of defection by stationing additional units in the city, but this often isn't enough, and if the city defects you lose the units too. Generally, the best strategy is to capture cities on the outskirts of their territory first, where they will have less culture output. Or be prepared to capture all their large cities in rapid succession. Also think carefully before sparing a civilization, as destorying the civ completely insures that your cities can't defect back to them.