Nice, cheap HR units to keep your cities happy. Though I usually tech in advance longbowmen and musketmen, and use them to fill my cities up to their happiness limit, so that I can have enough power on the graph and opponents don't attack me.
Unfortunately, even when I do that, I still seem to have 0.5 power compared to everyone, and eventually someone decides to declare war on me. Every city you ever have, you're supposed to build nothing but military units and no buildings, to keep up with the AI apparently.