A rts/roguelike in which you can control an adventurer or a whole dwarfen outpost in a randomly generated, persistent world.

(from the wiki) Dwarf Fortress is an ASCII game which includes both a roguelike adventure mode, and the more popular Dwarf Fortress Mode, which focuses on the creation and survival of a small dwarven settlement. It has a very steep learning curve, partly due to its ASCII graphics, but also due to the fact that it is one of the most complex games ever released.