I dont face many assaults in the game.I mostly handle these things stratecigally. I place my defensive armys so that they are actually very near the enemy cities.If enemy army sieges my City, i usually move the war in their domain by counter sieging one of their cities.This usually leads into that the enemy army breaks the siege and tryes to lift the siege from their own city,what gives me the edge of fighting an defensive battle and usually gives me a new city.
If this wont happen i bring army from the interior from my domain and attack the invading force,but this have not happened many times.