Dwarf fortress is a free 2D ASCII-based real-time strategy game. At first blush, it looks a lot like NetHack, which is arguably true. However, NetHack does not give you control of anywhere from seven to two hundred and fifty dwarves, with orders to build them...well...something resembling a fortress.
As any long-time player will tell you...dwarves are fucking stupid. Shockingly so, in fact. At least one of the two is almost guaranteed to fall in if you have them dig a pit. They can go crazy and starve while crawling around the kitchen stockpile, gibbering. They can go into strange moods and butcher a dwarf to make a pair of artifact pants out of dwarf leather. (And these are some of the more normal things that can happen.)