Rise and Rule of Ancient Empires

You don’t need to keep a large standing army around your empire,
especially in the early stages of the game. Cities have a defense
value even when there are no military units stationed in them.
Even if an opponent gets lucky and blunders across one of your cities
with an exploring unit, he probably won’t be able to take your
city…and even if he gets lucky and takes your city, your people’s
affinity for you will likely return control to you on the next turn.
I don’t even think about raising an army until I have located at
least one opponent. Instead, concentrate on building the improvements
that allow you to build more advanced units. Military units are cheap,
and you can raise a respectable army in 2 or 3 turns if you’ve built the
infrastructure first.
If you should discover an opposing civilization before it finds you,
send a small to medium-sized force to threaten at least one city.
Approach it from a direction other than directly toward your own
civilization. The idea here is to panic the other player into raising
an early army of weak units, and to mislead him as to the direction
of your own cities, while you continue to advance your infrastructure.
It’s also a good idea to station some pickets near your enemy, as far
from your own cities as possible, to keep his explorers away.
Don’t waste time or resources on internal trade. External trade will
help with conquest of opponents’ cities, but as a practical matter the AI
opponents will rarely maintain peace long enough for you to trade effectively.
Merchants are a valuable unit in head-to-head play, but have minimal
impact on a solo game.