Freeciv

Freeciv is a single- and multiplayer turn-based strategy game for workstations and personal computers inspired by the proprietary Sid Meier's Civilization series. It is available for most desktop computer operating systems and available in an online browser version. Released under the GNU GPL-2.0-or-later, Freeciv is free and open-source software. The game's default settings are closest to Civilization II, in both gameplay and graphics, including the units and the isometric grid. However, with a lot of multiplayer games being played in longturn communities, rulesets and additional variants have evolved away from the original ruleset. Freeciv is playable online on various public and private servers.