The creation of a fortress creates free villages. Doesn’t that open the doors for an enemy offensive?  
We consider the chances of enemies getting a lasting foothold in areas of your own tribe as quite low. In border areas or less densely populated areas it will be ease for enemies to become your new neighbours. But who would want to have a new village next to an enemy fortress? 
We expect that the newly created villages will be a tactical element that ensure more movement and fun in the game.