One thing that has really improved in this mod over vanilla is how the AI works, in warfare. They are a lot better & sensible but I do sometimes wonder if their priorities are self damaging on occassion. In present game am playing The Huns on Pangeoa map with little water. In renaissance age & only six civs left, with three vassals of myself & Byzantine vassal of Carthage. Despite both having many cities we don't have borders, being on opposite sides of the massive continent. I have advanced alot & border Byzantine, who border Carthage. Both civs have irritatingly been sending missionaries to my cities, with neither stopping when asked, so declared war on Byzantine. This is a world war as rest of civs are vassals & I currently ally all remaining CS except Monaco, who Carthage have as ally.
When declaring war I calculated the actions of Carthage & proved right. As carthage is naval power over half of his massive forces are naval units, which is no loss to me as majority of my cities are not on coast, & ones that are, are either unaccesible to him or hard to take. Mind you with two ships I wouldn't put up much of a fight. I knew it would be difficult to get a foothold due to Byzantine city layout, but Carthage ensured there was little opposiiton. Upon war their fleet advance on CS of Bueno Aries, whilst their army advanced on Melbourne. In next 10 goes I ransack through the Byzantine empire, whilst a solitary Carthage Longswordsman looks on. Admitingly Melbourne is likely to fall, & must praise AI in ensuring their ranged units don't die, but unfortuantely at the cost of their melee units. So the time they finally meet me they will have me on their borders with no protection.
The point I am making, which happens regularly, is the AI puts more priority in overthrowing some CS, than protecting an important vassal or their own lands. I know vassals don't take very kindly with their cities being attacked. Spain in a previous game got quite annoyed when Rome ravaged it, though in fairness had trouble reaching it due to mountains. Looking back though they were right as civs become vassals for protection not to see their masters go walkabout & only concerned with themselves..
I just ask that AI take the prioirty of their own lands & vassals over other things.