Many thanks
Mrwhiskerson
Go to the World tab and select "regions"
Get comfortable with the http://cs.elderscrolls.com/index.php/Regions(that article is for Oblivion but everything still applies)
Create a new region. When drawing the shape, make it surround your castle and make sure it also overlaps the main border region (BorderRegionSkyrim). Flag it as "Border Region".
Make sure you do NOT touch any other regions, as it will dump oodles of cell edits into your mod.
Alternatively, you could distribute an ini tweak with your mod.
Make a text file called "yourmodname.ini" with the following contents:
[General]bBorderRegionsEnabled = 0
Distribute it alongside your mod (NOT in the bsa I think).