Yeah, I was indeed distinguishing between battles on the open field and sieges, which I know were far more common and often less bloody, as everyone stood outside and waited for the people in the castle to starve and surrender. 

In fact, I think I mentioned the very same fact in the Arena chat thread a few weeks back when someone mentioned medieval battles.

I suppose the reason why battles were uncommon was that it was incredibly rare for two sides to want to fight and destroy each other, because it would rely on a situation where for some reason both sides were confident of victory, which just didn't happen. The smaller side would just run away (manoeuvring) until more favourable conditions were met. Where-as guys in a castle didn't really have the option of running away from larger forces, so the people ensured of victory could set things in motion and engage combat.

I didn't know that about Anglo Saxon England, though as is common I'm only really aware in any detail of the battle of Stamford Bridge and the Battle of Hastings, which were both foreign invasions, so I doubt rules of etiquette would apply.