I've made a small mod, adding 25 defense to every palace and 10 health regeneration, and after 2 games trial:
before of 9 ai, 3-5 died due barbarian activity
now of 9 ai, only 1 died (due barbatos or the dragon near the civ)
Change the CIV4BuildingInfos.xml, under each palace change:
<iHealRateChange>0</iHealRateChange>
to
<iHealRateChange>10</iHealRateChange>
and
<iDefense>0</iDefense>
to
<iDefense>25</iDefense>
However the infernal palace already had this bonus, so I didn't increased.
