I’ll just quote ForumDweller here:
Allow enemies to raid towns and villages and castles, but have them stop once the outer dwellings have all been demolished. While not very realistic, it’s the quickest and easiest way to retain crisis events without permanently destroying settlements. Furthermore, this approach is quite balanced because it robs players of valuable resources, but not to the point of no return.
oldschool rpg blog (in czech language)
http://2k6goblinu.blogspot.cz/