Bari campaign, mission 5 ( The Onrushing tide), halp!

I posted this on the STEAM forum but got no replies, perhaps because not many people have done the Bari campaign yet. Here's the situation -

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I've taken out all the Norman blacksmiths, the monastery, the woodcutters and the siege engines and my only remaining objectives are "defend Bari" and "Andreas must survive." The problem now is that the Normans are hitting Bari with 150+ man armies of paladins, halberdiers, 2-handed swordsmen, crossbows, trebuchets and rams, and all I have to defend are a handful of pikemen, cataphracts and Andreas himself. Oh, and a walled-in castle. So I garrisoned my tiny army inside the castle and did a good job cutting down any enemies who came close, but the problem is the trebuchets. I sent my cataphracts on a heroic suicide attack to destroy the trebuchets and they were all wiped out. Now I just have Andreas, Whenever he leaves the castle every enemy paladin and pikeman in the area, even those out of his sight, hone in on him like Andreas-seeking-missiles, so I can't use him to destroy anything. All he can do is sit in the castle.

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I assume you're supposed to lose this mission based on the fact that you can't get any villagers on this mission and you only get a tiny tribute amount to build a few units against a massive enemy army, but I survived for quite a long time already and took out around 300 enemies attacking Bari. How long am I supposed to survive to get a victory? Also, is there some trigger I'm missing somewhere?