Freeciv21 is a free open source turn-based empire-building 4x strategy game, in
which each player becomes the leader of a civilization. You compete against
several opponents to build cities and use them to support a military and an
economy. Players strive to complete an empire that survives all encounters with
its neighbors to emerge victorious. Play begins at the dawn of history in 4,000
BCE.

Freeciv21 takes its roots in the well-known FOSS game Freeciv and extends it
for more fun, with a revived focus on competitive multiplayer environments.
Players can choose from over 500 nations and can play against the computer or
other people in an active online community.