Background: I am running a tabletop game for some friends where they are in the middle of a war. I would like to figure out ahead of time what was the etiquette during siege, in case they end up there. The culture is a combination of europe and china and before widespread use of gunpowder.
I have been able to find a few things about the mechanics of a siege. Things like trebuchets were more to mess with the defenders, the main goal was to dig under the walls to make them fall, and that charges we see in the movies were only if they were short on food and couldn't keep the siege going.
However, I still have some questions about how the people related.
I'm not sure what in the way of special laws you could be thinking of; generally, like in any military situation, the commander of the works would have complete jurisdiction and could generally enforce his will on subordinates - unless, of course, they were so dispirited as to be mutinous. In all likelihood, life under siege would be a combination of toil and boredom, undercut by intense moments of fear. Damaged walls would have to be reinforced and countermines dug (if they had that capacity; it takes training to dig a tunnel underground in such a way that it will intersect another tunnel; expert miners were generally brought with besieging armies). If they succeeded in this, a hellish fight would take place between hunched and near-blind men.
Disease, hunger, privation, and loss of morale would be your main issues. Food has to be brought in, either by ship, pack animals, or "foraging" - that is to say, theft on a grand scale. Winter quarters of some variety must be constructed if the siege drags on.
That's pretty much the only way a besieged force is going to come out on top. Very commonly, an agreement would be reached between the respective commanders that if a relieving army did not come up by a certain day, the place would be surrendered. If forced to conquer the place completely, survival was unlikely (for anyone who did not look able to pay a ransom). Even if they held out until the situation was very desperate, last-minute negotiated surrenders were common.
It would certainly involve a truce. That could mean an exchange of letters carried by messengers under protection, or a face-to-face between the two commanders or chosen representatives. Bear in mind, pretty much anyone of officer rank is an aristocrat, and aristocrats tend to protect each other.
No idea. The only priest likely to be in a medieval European castle would be a chaplain, usually attached to a lord's household. In the case of castles without resident lords, I wouldn't think it likely there would be any within.