Out of curiosity, why stick with bows and crossbows? I'm a bit stuck on the idea of using primitive poisons to kill off PC's with stat damage (which, despite the fear of various amphibian god's wraths, is possible on many fronts). Why not use blowguns and poisoned tipped darts? Primitive tribesmen use it to great effect, and it would make the openings required to fire through a wall extremely tiny (2 quarter sized holes, one with a cone to aim, the other to see) disguised as part of a wall.

Eliminates effective means of retaliation as they have total cover and to jam anything through would require action against the wall itself first to make a hole. Dozens of pin pricks and saves, somebodies going to feel something, and 3 damage from a lucky crossbow is nothing next to 3 con damage from a lucky blow dart. And we haven't even progressed past tribal human technical abilities yet, let alone medieval.

Also note that this is not a tactic that would be devised to counteract any specific PC threat through GM fiat, and seems a completely normal way to defend their home against any variety of intruder.

People seem to forget that we've been killing each other for as long as we've been around, and been awfully good at it too, without the aid of anything technologically fancy. It only makes sense that in a DnD world, which is a million times more dangerous than ours, that something of a match to our intelligence would muster a surprisingly lethal defense against vastly superior forces.