Dwarf Fortress. Once you get past the fact that it's ascii graphics it is a remarkably complex game. You set up this elaborate fortress, get your economy going, etc all the while mining deeper and deeper into the mountain. You set up elaborate traps, drawbridges, retracting bridges, irrigation systems. You channel magma through the mountain and set up floodgates to flood key rooms when the goblins and trolls come to siege. Finally you reach the most precious veins of adamantine and begin mining it furiously creating all sorts of things from it - weapons, armor, goblets, tables, doors, thrones...
Then suddenly the game is over and you see this:
