If your browser supports HTML5, just go to play.freeciv.org to start playing.

Even on lowest difficulty, Artificial Intelligence of Freeciv will give you a run for your money.

There are many different clients available with different UIs and features.

In Freeciv you have to take into consideration terrain, resources, cities, economy, units, combat, diplomacy, governments, technology, wonders and many other things both for your and all competing civilizations.