When it comes to drawing a castle on a regional map then you'd best forget trying to get it properly to scale. Regional maps cover hundreds or thousands of miles and the actual scale of the castle would be less than one pixel so that's why we exaggerate...just to give the map some flavor and indicate what sort of style the castle has. It's meant to convey information and if you want to focus on the distance between things as being accurate then just use little dots. Now if you were doing a town map, then you'd have to have things fairly accurate so one pixel equalling one foot would give you no headaches.