Do you have a game you play over and over, when nothing else fits your mood? That game that is infinitely re-playable, somewhat the same every time and somewhat new and interesting?

For me it's the games in the Orcs Must Die series. But mostly OMD 3. I play the mode called Scramble, where you have to beat 5 rounds of orcs, and they get harder and harder as you progress. There are random things to make it harder (nerfs) and you can choose 1 thing per round to make it easier (buffs). I've probably played that something like 300 times now since I beat the main game + DLCs a long while ago. I go into a special mental state while playing, since I know it all so well, and just zone out for about an hour each time.

Patient gamers, what's your "I will keep playing this game for the rest of my life" fallback?