Great. His Hatness called a crusade to Mongol-owned Antioch. My Hungarians had been sitting idle for much of the game so far, as I wanted to take it slow and let the AI do some developping first. Now I thought "this is an opportunity to see how tough those Mongols really are". Of course, they were tough, but I was tougher... although I was aided by a Sicilian crusader army in the decisive battle. It was awesome: My full stack plus a Sicilian full stack against two Mongol full stacks - it was slaughter beyond belief (largest unit size setting!).
Anyway, I took Antioch with my King, who had been the leader of my crusade army (wanted to RP it historically there). He had had like six chivalry, so I had no prob controlling the city with his army still around 700 strong. Well, until the next turn, that is. A puny level-three assassin toasted my King, and Antioch went into unrest immediately!
I have managed to hold onto it, but only barely. Unrest is a constant problem, I was lucky I can hold races in Antioch (structure was captured from the Mongols). The Mongols have sieged me twice in the meantime, but the second time, I killed/routed a very large army of theirs, plus reinforcements, in a sally - cannon towers did a great job there. Still, there have been tough moments. Twice I had to withdraw sallying forces into the city, and the Mongols brought a few troops through the gates. Halberdiers posted immediately behind the gate took care of them, though. I won a resounding victory, and for the six turns since then the Mongols have left me alone. I have brought the Pope to declare another crusade - to Jerusalem this time, which is also Mongol - and one full stack of eager Hungarian crusaders is already close to Antioch.
Lessons learned: Bring more than one stack in the first place. Play the Hungarians or some other faction close to the holy lands. Bring a few spies and assassins to take care of enemy agents. Bring a few cardinals to quickly convert the populace. And take at least one more settlement close by - gives you so much more leeway!