Credit: Original article with some input of my own on Windmill shade
Buildings
Buildings cast shade (up to 3 tiles) that will slow down crop production for any fields nearby. The taller the building, the further the shade extends, so make sure that your farms are safely separated from your buildings (at least 3 tiles in between) to ensure efficiency.
The Wheat Field optimally takes 20 seconds to regenerate. Since the highlighted Wheat Field is placed 2 tiles away from the Windmill, it takes about 80 seconds to regenerate instead. The Wheat Field at the top takes the maximum 160 seconds to regenerate, as it is 1 tile away from the Windmill.
This tree is so shaded it’s hopeless!
Mountains
Mountains cast shade on the first 5 rows of tiles in your town. When you place your town next to a mountain, be prepared to turn the mountain side into an industrial area that focuses on mining. You’ll probably want to build your farming areas on the other side of town to avoid the mountain shade.
This screenshot was taken from here – see the industrial area at the left centre and the crops at the bottom left.
Remember these efficiency slowdowns while building your Town!
Keep in mind that while shade and wind block may slow your production and efficiency, they will never ruin it. Your Windmills will still craft even if they are placed directly next to one another in a row; they will just take the longest amount of time possible to produce each round of goods. This is indicated by the red production needle that is displayed when your Windmill is milling a good. Your crops will still grow, but they will take much longer.
Basic wind block and shade placement strategies will become second nature to you the more you play Town Star. Choices made early about the structure of your town will ultimately be what separates a functional town from a winning town.