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How do I slow it down?
Do you mean the turns go by too fast? Or the game seems too responsive or jerky?

For the first one, I don't know of a way to make the months go by more slowly. For the second, you can try in the DOSBox config file to lower the game cycles.
If you're talking about castles 1, just go to settings/options and change the speed to laboured.
I would agree that the mouse in Castles 1 is quite sensitive when you're trying to adjust wages and labor forces, and I've mis-clicked several times.

I believe he's referring to the march of time in the game, though. Even on "Laboured" I can see how you may think the game won't stop, and in fact it won't.