#2 a town with 650 defenders will defect to 200 top tier troops. IFF there is no lord present. With lord present there is a fight more often but not always. If the odds are bad the castle/town surrenders and expells the lord and his army so you can fight them...
Yeah, this is a problem, I think. I don't know that a castle should really surrender much at all if there's more than 50 guys in it. A castle garrison of 200+ really shouldn't surrender, period. I had about 200 guys, as Gerard noted, and I couldn't find a castle/town that would fight, unless the lord(s) in it had lots of troops. That's just weird, if you ask me. Just waltzing around, taking every castle by intimidation didn't even work every time for the Mongols. Hey, that's a nice castle you got there, can I have it? Sure, come on in, let me open the gate for you. I would be quite surprised to learn that this was all that commonplace.
I have a house rule that I won't ask for surrender unless the castle was just taken and couldn't be expected to have any troops in it. Auto-surrendering of castles is also bad, strategically, because even if that garrison is going to lose (and how often does the AI win anyways, once you're up and running?), it can still inflict substantial casualties, even of top-tier troops, meaning you can't just blitz Brytenwalda. Even if you only lose a few dozen guys at a time, the top-tier guys are harder to replace (and should be even harder to replace, IMHO), so each castle gets harder, and the casualties go up, you will likely run out of steam, and out of the guys needed to not only take castles but to defend them from the counter-attack.