I think you need to get 15 useful moves in a row to get the windmill ma…

I really cannot get the bonus for the mill either!

15 Useful moves? What do you mean, 15 moves without moving one wrongly?

I kept restarting as I thought that I had to do something impressive with the sacks of grain.... Lots of 3's of just that......

You have to get all of the buildings, for a total of 20, and when you get that, you'll get a chance to build Mount Olympus. If you build that (if you complete level 100 after building all of the buildings) you win.

By "15 Useful Moves" it is either actually referring to 15 moves in a row in which you break a chain/blue tile, or it refers to the times when you break a trio (or more) and that starts off a chain-reaction of other moves. I've gotten the Mill bonus a couple times and never actually paid overmuch in the way of attention to how exactly it happened, but that's what I came up with.