Freeciv

Freeciv is a free turn-based empire building game in the style of the Civilization series. You found cities, manage production and trade, research technologies, build wonders, and expand through diplomacy, conquest or a peaceful race for science.

You can play alone against computer players, with friends on a local network, or on public servers. Several rulesets change the pace and the rules, and a web version is also available. Games can last many hours, and long games can be saved and continued later.

The game is open source under the GPL, with no ads, no accounts and no purchases. It suits players who like long strategic plans.