Stupid AI!!!!!!

The other day I was playing one of my personal favorite games called Medieval 2 Total war. I was playing the campaign mode which is basically a game mode in the game, that you choose a faction, which is basically your country or principality and you start off on the "campaign map". Well what game basically has two aspects during the campaign. One is the campaign map, and the other is the battle map. The battle map is where you get to command your toops and you get to see the epic battles go down. On the campaign map you get to manage your cities, taxes, religion, ect. You also get to choose to send armies to places to either defend your cities or siege the enemy cities to expand your empire. Well I was at turn 300 and something, which is a long way into the campaign, considering each turn is a season. Well anyways one of my enemies decided to siege my fortress which was my last line of defense before my capital. There were two AI enemy armies attacking, and then my army, which I controlled. And then helping me out was another AI army. During the battle the enemy armies managed to get through the first set of walls to my fortress. What happened next made me rage quit. The AI army helping me defend just froze up and allowed the enemy armies to walk right past towards my second, and final wall defenses. Long story short, I lost the fortress and because of the stupid AI my capital was vulnerable.