Not sure if it's a cheat, but rather using a tactic that takes advantage of the AI logic. Though, to be honest, it doesn't always work, as sometimes the AI sieges in short order depending on the castle's defenses and if the AI has siege engines.

Out of places where I did this (Leon, Castille, and Syria), Syria fell quickly, Leon fell twice and Castille fell and then was retaken (happened when the Spanish attacked me again in Navarre and Aragon, nice victories for me too).

I am still surprised by the AI on occasion in battles, and I don't always get easy battles, still challenging. Something that makes MTW a very enjoyable game, its replayability.