mini metro is free on android rn! (not sure about other platforms but probably worth checking)
its fun but can be frustrating but a good way to pass time imo!
@spacekatia mindustry is really cool, I found it even better to play on a desktop. Also for the phone I can additionally recommend Shatterd Pixel Dungeon, a very challenging rouge-like https://f-droid.org/app/com.shatteredpixel.shatteredpixeldungeon
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