Sail south down the western seaboard and attack the invading army. Alexander III may have a strong army behind him, but you have the support of the kinsfolk of the Isles. They will fight for independence from the Scottish Crown and ultimately the Anglicisation that would result if they lose.
Use Duguld's influence to get the Gaelic lords on side. If you can unite
the Gaelic and Norse forces, Alexander III would have his work cut
out to defeat you.
Sail down the western seaboard to confront the situation. No Norwegian king has personally visited since 1098. You can meet with Alexander III and hopefully come to some sort of arrangement. Surely there is a peaceful way to settle this volatile situation?
Which card will you play?