Rather than get into my specifics, how many of the following do you normally have before engaging in voluntary war in the first few hundred years of AD?
There's no real way to quantify this in advance since every game, and every war, will differ.
* If you're contemplating an attack, accurate intelligence is key. If you have a religion founded get a priest to spread the religion into the target city(s) ASAP. Get Line of Sight any way you can so you know what you're up against. If you have Open Borders send in an Explorer/Scout to see how the defenses are laid out. If it's an ocean city remember that Caravels can enter cultural borders even if you don't have Open Borders.
* Once you know what you face in the way of defense you can guesstimate what you're going to need to take that city. Once you figure you have enough to take the city, add a few more units just in case.
Tip: With one of your units selected (and well outside actual attack range) hold down the ALT key and right-click and hold the right mouse button down on the target city. You'll get the combat odds for that unit attacking that city. You can do this even when you're not at war with that civ and nowhere near close enough to attack.
* You MUST bring enough to do the job on the first try, which means over-estimate the number of units you're going to need.
* Bring 1 suicide catapult for every defensive unit. You won't need them all to suicide, but a 1:2 ratio of suicide catapults to defenders allows you to bring down the defensive bonus swiftly
and soften up the defenders for your attack while retaining a reserve for dealing with the counter-attack SOD or with other city attacks. (The above might not make sense, so here's a fer-instance: If the city is defended by 6 units, bring 6 catapults. Use all 6 to take down the defense bonus then have 3 cats suicide assault the defenders. The 3 cats will almost certainly die - but in doing so they make the
real attack by your macemen/swordsmen/whatever a cakewalk - and you have 3 left to deal with the SOD or take another city)
* SOD: Stack-Of-Doom. In the opening phase of the war the AI will counter-attack you with everything it can bring to bear. Your assault must bring sufficient units to hold what it takes. Bring defensive units with you (Archers/Longbowmen) Do not make the mistake of underestimating what the AI will throw into the fight. If you bring just enough to take the city and have one or two un-damaged defenders in the city after you take it... good luck keeping it.
* Make a note of where all
your workers are before you attack. Make certain none of them are within range of a quick snatch by an enemy mounted unit when you declare war.
There's lots more, but half the fun is figuring this stuff out by trial and error. Have fun
