Early Game 
As Rome, I usually start with researching Mysticism first. My rationale is that I don't build workers early anyway, so getting the techs to improve resources isn't as essential. I build an early shrine for religion, and then spam warriors (barbs are rough). If I have a location heavy with resources that can be connected by a certain improvement, I go for that improvement. Otherwise, I head straight for Bronze Working to find the Iron. If there is a lot, and I have a neighbor nearby, I gear up for war. I get Wood Working and Iron Working (in that order, because Ballista have to be slow built while I can just upgrade my Warriors, so they actually pop around the same time), adopt Monarchy for the military bonuses, and attack.
If I lack Iron and/or I don't have a neighbor nearby, I get the domestic techs, and adopt Imperialism for the fast expand possibilities. In these situations, I adopt Theocracy for the growth and happiness.
Early-Mid Game 
By now, I've met my neighbors (scout, scout, scout), and I get a general sense of how to play out the game. At least one civ is taken out by this point, so the conqueror is high up on the scoreboard. If I have many bonus resources around, I emphasize the Economic Tree for the Monopoly Buildings; don't underestimate the power of these boost. If I don't have many resources, I focus on improving my science and happiness. Either way, if I have unfriendly neighbors, I keep a large standing army; if you get attacked without one, you *will* lose some cities. Depending on the state of the world, I'll either switch to Democracy (usually done if I picked Theocracy) for the growth and science boost, or I'll stay with Monarchy for the military. If you are thinking about warring, get up to cannons; cities have very high attacks, and you do not want to lose your siege weapons the turn that they get in range.
Mid-Late Game 
War, plain and simple. WWI Infantry have a much improved attack over Riflemen, as does Artillery. Get up your unit producing buildings fast (Combined Arms), and then crank out the heavy machinery. If you have a decent jump on the other AI in this area, you can probably take out one or two civs. Your main concern will be happiness, and Global Opinion of you; if you DOW, you will not have any friends left (except for your Tanks
). Carve yourself a nice little area, and hold tight. Don't be too aggressive though, as AI DOWs from multiple directions can be the end of you. I only suggest getting Autocracy if you are really in trouble; yes, the military bonuses are top notch, but you sacrifice the opportunity to spur on technical advancement.
i want to get a feeling of how many units players usually have in each of the eras? So it would be really helpful if i can get a bit of feedback on this eg:
