The Lonely Fortress

From publisher blurb:

A recent uprising amongst the local tribes of the far north has upset the careful string of alliances that the Bezian Empire has worked so hard to forge. The uprising was, of course, put down harshly and easily by the Imperial Army. But the peace has been broken. Without the Imperial Peace, the locals tribes have resumed their perpetual infighting. And one of those tribes has decided to built a fortress on its border in order to aid in their defense. ...A fortress, in Imperial territory, occupied solely by one of the conquered tribes. That is something that the Empire cannot allow.