Ranked in these QuestionsQuestion Ranking
Pro Intelligent AI
Even on lowest difficulty, Artificial Intelligence of Freeciv will give you a run for your money.
Pro Excellent depth
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.
Pro Free, open-source & does not require a plugin
Pro Observe public games
You can observe public games currently being played and even take control of an AI civilization to jump into the fight.