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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. Dwarf Fortress is completely free.
Before you play, you must generate a world to play in, which persists until you create a new one. World generation can be time consuming, even on modern computers, but be patient. It's worth it.