The city of Warchalking had a water mill.
They presumably have signamancy that includes a large spinning machine. Mills are probably used to grind grain and provide a bonus to an associated farm, but they might be used to provide power for other purposes as well.
Real World References
At the outer reaches of plausibility, "mill" in deck-building games such as Magic: The Gathering or Hearthstone means to deplete your opponent's deck more quickly than normal, hoping to run them out of cards.