Widelands is a free, open source real-time strategy game with singleplayer campaigns and a multiplayer mode. The game was inspired by Settlers II™ (© Bluebyte) but has significantly more variety and depth to it. Still, it is easy to get started through playable tutorials.
Widelands offers a unique style of play. For example, a system of roads plays a central role in your economy: all the goods that are harvested and processed by the tribe must be transported from one building to the next. This is done by carriers, and those carriers always walk along the roads. It is your job to lay out the roads as efficiently as possible.
Another refreshing aspect of the game is the way you command your clan. There is no need to tell every single one of your subjects what to do - that would be impossible, because there can be thousands of them! Instead, all you need to do is order them to build a building somewhere, and the builders will come. Similarly, whenever you want to attack an enemy, just place an order to attack one of their barracks, and your soldiers will march to fight. You're really a ruler: You delegate in times of war and in times of peace!
Widelands offers single-player mode with different campaigns; the campaigns all tell stories of tribes and their struggle in the Widelands universe! However, settling really starts when you unite with friends over the Internet or LAN to build up new empires together - or to crush each other in the dusts of war. Widelands also offers an Artificial Intelligence to challenge you.
In the end, Widelands will be extensible, so that you can create your own tribe with their own set of buildings, create new worlds to play in, and even create new types of worlds (who says you can't build a settlement on the moon?).