Freeciv
A free, open-source empire-building strategy game inspired by Civilization, playable in single-player or online.
Freeciv is a free and open-source turn-based empire-building strategy game inspired by the Civilization series. Players guide a civilization from the founding of its first city through technological eras, competing for dominance in a world map.
The game offers a deep tech tree, government systems, diplomacy, trade, espionage, and both peaceful and military paths to victory. It can be played in single-player against AI or in multiplayer with up to hundreds of players on a shared server.
Freeciv is licensed under GPL-2.0 and is completely free. It is one of the oldest continuously developed open-source strategy games, with a classic client plus a web client that runs in the browser, and a large community server network for online play.