[Endoperez]

Pages 41-42 of the manual explain fortress resource collection in detail.

When a fort is built, provinces produce double the resources they produces before the fort. The resources collected from the neighbouring provinces are first collected from the potential resources the province would have if it was castled, so 50% of a province that has 13 resources is 50% of 26 resources, or 13. Manual has pictures and other things and explains what happens with two adjacent castles.