OK , here we go. I've been using the Inca's like crazy lately , now they're my no.1 civ. > great army good economy.
Here's what I do , mass as many Quechua ( they're so damn cheap! 15 hammers ) basicaly every turn you can crank one out , you should quickly set up 3 cities , all with barracks to give you that extra edge , and keep a few villagers chopping like mad for quick extra hammers ......
(and by the way , you can bring your villagers far out to chop , because your Quechua are effective against barbs and animals.)
You have a 2 choices > early on its easy to completely wipe out your nearest civ. ~ but the down side is you'll get a bad reputation ( trust me ... it happened tome already ! with the Inca )
OR even better
dont stop massing Quechua , dont worry about maintenance costs for so many units > Vassalage will cover that ( and also Inca are great at generating commerce )
Plus use Hereditary Rule , and you'll have 1 extra of happiness in your city for each cheap Quechua guarding it , that only costs 15 hammers for 1 happy face and city defence ~ not bad.
Then when you feel ready ( when the AI starts a war with you ) let the hordes loose ! even if they have more advanced units large numbers will overcome anything. If you're clever add some Axemen , Quechua are only a little weak against swordsman.
I would rather keep my Quechua for as long as possible, in fact I would trade my iron away just so they're not upgraded automaticaly.
Never underestimate the power of cheap massed troops , it also makes it hard for the enemy to capture your city quickly if you have large numbers inside, itgives you more time to react.
At the end you easily end up the winner by far because of a strong military force and a good economy ( if you know how )