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Open ended city-building simulator. Minecraft meets The Sims, with a difficulty level that can only be expressed through swear words.
Slaves to Armok: God of Blood Chapter II: Dwarf Fortress is a part roguelike, part city building game. It generates a high fantasy world using fractals and mathematical techniques so that each world is completely different. Like most roguelikes, Dwarf Fortress is text-based but don't think that means it's light on computer resources. DF is the most complicated and in depth game ever. Tarn and Zach Adams have worked on the game since 2002 and plan to work on it for another 20 years before it comes out of alpha.
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Pro "Starter Pack" offers a streamlined introduction
The DF Starter Pack, formerly Lazy Newb Pack, is a collection of community-created enhancements to the interface and visual presentation of Dwarf Fortress. Find it here.