This is the most discussed topic on the forum. Welcome newbie 
To be a bit more serious, The Frog (AI designer in FE) claims that the monsters doesnt know the difference between AI and human players. When a monster raze a resource its supposedly by random chance that the monster walks on it.
Lately I have seen that empires have been exterminated early game, indicating that they have been killed by monsters. Also remember that Tarth has some special protection against monster attacks and that there is some "monster repellent" thing that heroes can have.
