The feature "Increase expansion radius for fortresses" enables you to attach castles to your fortress center from greater distances.
As soon as you created a fortress center, you see all possible habitats in the predefined attachment radius. Now you have the possibility to increase this radius by a certain value (+3), so that you can attach habitats from greater distances. The usage of the feature requires magic potions (500)
The increase of the attachment radius is just temporary (12h), so have just have a few hours to start the attachment process of a chosen habitat that would usually lie out of range. Once the attachment process is started, it will not be canceled after the runtime of the increased attachment radius has ended.