yes setting destroy_empty_forts to false will make them permanent...if its true they will disappear if nothing is in them for 1 turn....also this is what my devastation looks like;
Code:
<fort_devastation_distance uint="20"/> <!-- maximum movement point extents for determining devastation tile targets for forts -->
<army_devastation_distance uint="20"/> <!-- maximum movement point extents for determining devastation tile targets for field armies -->
<fort_devastation_modifier float="2.0"/> <!-- num devastated tiles modifier for forts -->
<army_devastation_modifier float="1.0"/> <!-- num devastated tiles modifier for field armies -->
and i get a lot of devastation in my game so maybe try half of these numbers and see if it's what you want...if not then gradually increase to find a level you are happy with...