Freeciv is a free and open source empire-building strategy game inspired by the history of human civilization. The game commences in prehistory and your mission is to lead your tribe from the stone age to the space age...

This is huge bigfix release of Free and open source empire-building strategy game inspired by the history of human civilization. FreeCiv commences in prehistory and your mission is to lead your tribe from the stone age to the space age.

In the world of turn-based strategy games, Civilization rules the roost. Or at least used to. I personally have spent a lot of hours over the years playing the original series (although I’m not much of a player any more, and never really was very ‘good’ by any decent standards).

Unknown Horizons is a FREE and open-source, real-time 2D strategy game for Windows and Linux. The game emphasis on economy and city building:

Expand your small settlement to a strong and wealthy colony, collect taxes and supply your inhabitants with valuable goods. Increase your power with a well balanced economy and with strategic trade and diplomacy.

This game has become very popular. It is a first person shooter game with elements of real time strategy. Free to download and open source.

FreeOrion is a turn-based, futuristic science fiction computer strategy game for Windows, Linux and Mac OSX. The purpose of the game is to lead one of a number of races to dominate the galaxy through a combination of diplomacy and conquest while developing technology, exploring and colonizing star systems.

Warzone 2100 is a free, real-time strategy and real-time tactics hybrid computer game for Windows, Linux and Mac OSX which offers campaign, tutorial, multi-player and single-player skirmish game modes, unit designer, unit experience system, and everything you would expect from a modern RTS game.

0 A.D. (pronounced "zero ey-dee") is a free, open-source, cross-platform real-time strategy (RTS) game of ancient warfare. In short, it is a historically-based war/economy game that allows players to relive or rewrite the history of Western civilizations, focusing on the years between 500 B.C. and 500 A.D.

After recommending those excellent real-time strategy (RTS) games for Linux, let's move on to this other type of strategic gaming referred to as turn-based. A game is considered turn-based if the game flow is divided into well-defined and visible parts, called turns or rounds.