Dwarf Fortress

Developer: Bay 12 Games

14 years in the making and with decades still to go, Dwarf Fortress is one of the deepest and most complex best classic PC games ever. The fact that it doubles as a great free game is just a bonus. The open-ended gameplay tasks players with establishing and running a colony of dwarves within an auto-generated world with its own unique history. The original text-based graphics can be hard to parse (although modded alternatives are available) and the punishing nature of the world can be a turn-off, but persevere and you’ll be met with a hugely-immersive experience with a level of detail hard to match in any game that you don’t play with a D20.