Resilience

I completed the map, starting with the "Veteran" difficulty level, playing as the Necropolis faction. Starting from Biara, no enemy hero dared to attack me in the castle (although some of them were clearly stronger than my hero). Question to the map author: is this a feature or an AI defect (exploit)? Game version: 3.1 + a lot of mods (including a mod to enhance AI - Aiadv v.3.11).