Overexpansion of your economy,
especially during the early game, is
lethally dangerous to your empire. Pay close attention to the limits your civics place on you, and
don't expand beyond that point without a
very good reason - a must-have resource, or
possibly a chance to block a rival empire. A fortification may be better for that last purpose, though - it will allow you to claim territory without tanking your economy.
Later on, those limits aren't quite so hard - in my current game, I'm running Monarchy, which allows me up to fifteen cities before the penalties start kicking in, and my empire is sitting at nineteen cities. Despite the penalties - 12
in each city because my people are upset about me "overexpanding"! - I only had two cities actually have
any level of unhappiness, and a couple of new buildings in each of those cities got rid of that. I'm still going to hold off on any further expansion until I get some new technological developments; while I've got things under control at the moment, a reckless building spree could easily cause me more problems than I care to deal with right now. I'll probably switch to Republic before long, which will allow some more expansion (maybe three to five more cities), and then I'll settle in to develop the cities I already have until either Democracy or Fascism becomes available. (No limits on the number of cities!
)