Reading ASOIAF, I've never been quite clear on who the soldiers in armies are, and where they come from. Meribald's speech in AFFC implies that soldiers are pooled from the smallfolk:
Broken men are more deserving of our pity, though they may be just as dangerous. Almost all are common-born, simple folk who had never been more than a mile from the house where they were born until the day some lord came round to take them off to war.
(Emphasis mine.) But this doesn't say a huge amount. Is it indiscriminate - is every fighting man within the lord's lands taken as a soldier? Is it voluntary, or are soldiers conscripted? Do all able-bodied men within the families of the lord and his bannermen fight? I'm looking for more general information about how armies of Westeros are formed.