Recs.
Updated
Open ended city-building simulator. Minecraft meets The Sims, with a difficulty level that can only be expressed through swear words.
Not yet available on Steam.
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.
SpecsUpdate
Pros
Pro Massive randomly-generated worlds persist across multiple games
Estimating one meter per tile, the smallest possible Dwarf Fortress world is roughly 13 kilometers on each side, and about 400 layers deep. The largest world is about 197 kilometers (123 miles) on each side. Each world has its own randomly-generated history with thousands of notable figures, warring factions, and numerous settlements of various races. Any number of fortresses and/or adventurers can be played in a particular world, though only one at a time.
Pro Renowned for depth of simulation and emergent gameplay
The ultimate goal of Dwarf Fortress is no less than a total fantasy world simulator. Its simulated systems include geology, metallurgy, fluid pressure, temperature, combustion and phase transitions (melting, evaporation, etc.), shear and tensile strength, ballistics and gravity, anatomy and tissue damage, growth and maturation of animals and plants, emotional states, life goals, and alcohol dependence.
Cons
Con Controls are complex and often counterintuitive
New players are advised to read the Quickstart Guide.
Dwarf Fortress takes a lot of patience to learn. Even something as basic and early as world creation might take a guide to understand the first time around.
It doesn't really matter how experienced of a gamer you are; if you try to play this game without a guide, you're going to have a hard time.