In the first siege, Denmark attacked my Russian Citadel at Hamburg. They had more heavy infantry than my garrison, and I had no cavalry or siege equipment with which to damage their siege gear. When the battle started I received the dread message that an enemy spy had opened the gates. So I prepared myself for certain and terrible doom and drew up my Boyars in front of the gates. Well, to my utter amazement the enemy totally ignored the gates and rushed their entire contingent of about ten troops up on ladder onto my walls, right into the loving embrace of two units of heavy infantry. I was dumbfounded. The castle was theirs for the taking, but the AI dropped the ball and allowed it's troops to be slaughtered.
This is a Known Bug.

So the bodyguard unit, unsupported except by a few units of peasant arches, took on three times it's number of elite heavy cavalry and was slaughtered to a man, leaving my general standing, to my disbelief, victorious and alone on the courtyard behind the gate.
3 Points here:

1. The Bodyguard has 2HP each so they are nearly as good as double their number of cav.

2. They have highier attack and nearly the same defence as the War Clerics so they are slightly better individually anyway than the Clerics.

3. generals have VERY fast and powerful atack animations and can thus kill 5 10 or more enemy with ease before dying.

You still did well, but it isn't inmpossibile to have this happen IMHO. Any expiriance his unit had would have only helped here.