A freeware colony/survival simulator Video Game with a really intricate simulation engine. It's nearly 20 years old and has a Reputation for being difficult or unforgiving (see Losing is fun!), for being inscrutible (it's a Keyboard Game; people say it's ascii art, but it's a very simple low-resolution SDL tileset engine where many objects but not all are represented with colored letters), for being poorly coded (the simulation is a single threaded CPU intensive system so the more successfully you play the worse the game performs).
After all this time, though, it's been tightened up, the quality has improved and in -ish there was a public "premium" release on Steam with graphics, mouse support, performance improvements... it even runs decently on SteamDeck. This version is something that I have been having a lot of fun with recently and hope to keep putting time in to.
it's an incredible engine for Emergent Storytelling and it's just fun to build a cool water-powered automation and see your dwarves happily exclaim that they were releived to behold a finely made chair next to a waterfall. While Rimworld is very combat and action-focused, DF spends a lot of time modeling the inner-worlds of your citizens, they experience trauma and are hardened by battle, they fall in love and care for each other, they want to consort with their fellow craftsmen in rum-filled guildhalls. You don't instruct them but you provide general orders and your little friends carry them out, or choose not to.