0 A.D. is a free (as in freedom) historical real-time strategy game which tries to portray some of the major civilizations over the millennium of 500 B.C. to 500 A.D. and thus has a great variety of civilizations you can choose from, such as Athenians, Macedonians, Persians, Romans, Spartans and many more. It is comparable to Age Of Empires and features epic battles with very large armies, either against an AI or in multiplayer matches. In fact it actually started as a mod concept for Age Of Empires II: The Age of Kings, but evolved to a standalone game with it's own engine, Pyrogenesis. It is still considered alpha and is not feature-complete, but already has a small and active gaming community. You can hang out in the gaming lobby and challenge other players. View a YouTube demo here.
emerge --ask games-strategy/0ad
Dune Legacy is a free remake of the classic Dune II with reworked graphics, improved gameplay, AI and controls while still maintaining the original game flair. It also features multiplayer and a map editor. You need the original game files to play (see the README or the complete list). View a YouTube demo here.
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FreeCiv is a free 2D turn-based strategy game inspired by the Sid Meier's Civilization series. The gameplay is like that of the Civilization series, in which the goal is to lead a civilization from 4000 B.C. until however the game ends. The player can discover technologies, conduct diplomacy and/or wage war, build cities and many other things. The game may end when a set year is reached, when one civilization has eradicated all the others, or when one civilization has accomplished the goal of space colonization. These are different win conditions that may be enabled or disabled. This type of customisability of the gameplay rules is characteristic of FreeCiv which has a list of settings that are probably overwhelming to new players but very rewarding once you get the hang of the game. It is a very deep game with very deep game play. FreeCiv has networked multiplayer (LAN or Internet). It may be played in several different clients. In Portage, we have the GTK2 and the SDL clients (use gtk and/or sdl USE-flags), of which the GTK2 one is arguably the most pleasant. The game may also be played online in your browser using the HTML5 version. View a YouTube demo here.
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Hedgewars is a turn-based strategy, artillery, action and comedy game, featuring the antics of pink hedgehogs with attitude as they battle from the depths of hell to the depths of space. Each player controls a team of several hedgehogs: during the course of the game, players take turns with one of their hedgehogs. They then use whatever tools and weapons are available to attack and kill the opponents' hedgehogs, thereby winning the game. A large variety of tools and weapons are available for players during the game. Hedgewars features the standard artillery-genre classics (such as Grenade, Cluster Bomb, Bazooka, Shotgun), but also boasts a host of other weapons quite unique to it alone. View a YouTube demo here.
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MegaGlest is a fantasy themed real-time strategy game, cross-platform and free as in freedom and gratis. It's a fork of Glest, but with an active community and many gameplay and art improvements. You can play as a single player or host multi-player network games (up to 8 humans or bots). Thanks to easy modding with libre software tools, several differently themed mods and (MegaGlest engine based) stand-alone games are available. Similar games include Warcraft III or Age of Empires. View a YouTube demo here.
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OpenRA is a free real-time strategy game engine supporting early Westwood games like Command & Conquer, Command & Conquer: Red Alert and Dune II. It's focused on multiplayer with an active community. Modding is also supported. Original game files will be downloaded when you start the game as they are freeware. View a YouTube demo here.
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UFO: Alien Invasion
UFO:AI is a free strategy/tactics game in the tradition of the X-COM game series in the 1990s. An alliance of the countries on earth tries to stop an alien invasion and must counter enemy forces with advanced technology and hidden goals.
The game includes strategic planning and aerial pursuits in real-time and turn-based combat scenarios between your soldiers, aliens and civilians. It also features a multiplayer game mode for up to 6 participants. View a YouTube demo here.
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Warzone 2100 is a free (as in freedom) real-time strategy game, in which you command the forces of “The Project” in a battle to rebuild the world after mankind has almost been destroyed by nuclear missiles. The game offers campaign, tutorial, multi-player and single-player skirmish modes. An extensive technology tree with over 400 different technologies, combined with the unit design system, allows for a wide variety of possible units and tactics. View a YouTube demo here.
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Battle for Wesnoth
Battle for Wesnoth is a fantasy turn-based tactical strategy game. You build up your own army during a match, improving the level of your troops while gaining control over the map. There is a large pool of units to pick from and it is very important to pick the right army for the job, depending on the enemy, the terrain and the task. Wesnoth has many different sagas: you can fight to regain the throne, step into the boots of a young officer, lead a brutal quest to unite the orcish tribes, etc. It features about two hundred unit types, sixteen races, six major factions and hundreds of years of history. You can make your own custom units, create maps with the built-in map editor and write your own scenarios or even campaigns from scratch. Of course it also features multiplayer battles where you can challenge up to 8 players. View a YouTube demo here.
emerge --ask games-strategy/wesnoth