Widelands is a libre remake — with further development — of an older RTS game Settlers II, focused mainly on building a sustainable and efficient economy, and a little on warfare. There is a variety of resources and buildings, and a player can apply different development strategies.
The first stage of playing it boils down to figuring how it works, what resources and buildings are needed, in what order to build them, etc. Then it’s mostly about perfecting the same strategy. Well, not very different from what one does in most of the other games: acquiring a useless skill, and applying it then.
There’s a lot of maps, some campaigns, and multiplayer is present over both direct connection and a game server.
The game is implemented quite well: as in The Battle for Wesnoth, the graphics here are dated, yet generally it is nice and polished. Though its build process relies on custom shell scripts, but at least they work – so building from sources should not be a problem.