Civilization 4: Vista installation has “install for all users” greyed out

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.

Constructing a building you'll never use is a waste of production. You're much better off switching to wealth in an economic city, for example, and science in most other cases. Of course, you may decide that you're done building up and turn all cities that have enough production to have finished building specialized buildings into military cities so that you can quickly crush the rest of your opposition, thus creating cities with dual specialization. Do that only if it helps you win faster than producing straight wealth, science, or culture.

Military (i.e. production) cities should switch to science in peace time (produce a library first) to help finish the game faster if you don't want to win by wiping out everybody.

Note that a factory is a must in a military city: you want to be able to produce modern units in 1-2 turns so that you can quickly react to threats, if necessary.

Also note that if you have time to build every single building, you should work on winning more quickly before moving up difficulty levels.