I've been playing the campaign and armies have amassed but I can't pin down the specifics of what needs what to generate them. I've just built and expanded like a plague and eventually the army begins to form. I build iron mines, coal mines, iron smelter, weapon smith and barracks. All within close proximity to each other. Early on I have fish being sent to the mines. I have people milling about that can be converted. I've set it to build weapons and recruit people to a particular martial training, but it only seems to convert 2 or 3 people, or the weapon smith stops making weapons. What am I doing wrong? What's the mechanic I'm not accounting for?

Thanks.

You must be in shortage of available settlers. Seems like everyone else is tasked to be a carrier, builder or in a workshop. Build a residence or two and you should be able to train more soldiers.

It's very simple. You need iron and coal for your weaponsmiths to make weapons and a barrack where free settlers bring a weapon and become soldiers. The more weaponsmiths you have, the faster you will build an army. I found out that with 4 weaponsmiths you are already a formidabble force. Don't forget to build large residenses, because you need free settlers to carry all the resourses around and become soldiers. With 4 weaponsmiths, you will make soldiers really fast. Also, fish is for gold and gem mines only. Iron ore mines need meat, and coal mines need bread. And lastly, I usually don't bother making archers and spearmen and make only swordsmen. Send 50 of them with good power level (you need to have gold stored), capture tower, destroy it immediately. A.I. gets confused. Capture next tower, rinse and repeat.
To become untouchable, explore edges of map, if you find an island you can build a tower on, build one, put a soldier in it then destroy all mainland towers. Now the A.I. can't atack you (unless they are really close to you and will expand into your territory, so keep that in mind)

Mine plenty of iron and silver and convert those into iron bars in your iron smelters. These iron bars go to the weaponsmith along with another bucket of coal to be made into weapons. You need to have lots of free settlers around to amass an army so build plenty of residences. Your weaponsmith will make tools which will then be taken to the barracks where they'll lay on the floor waiting for you to recruit soldiers (if you've set it to manual, which I do until I need a tonne of soldiers). If you then switch it back to automatic, and you have 8 settlers free (and next to the switch of Automatic/Manual you should have a number telling you how many are available to be converted to soldiers) then 8 soldiers will be created using those 8 weapons. If you have a number of iron smelters, weaponsmiths and barracks, and lots of residences chucking out free settlers, you'll be able to amass an army in no time. It's all about the production line.

Thanks. I appreciate the replies. I'll give these a shot if I get back to playing the game. I ended up just reinstalling CIVIII and adding a mod.