[@Irredeemable] Given what I'm presuming is the current state of technology relating to gunpowder, I doubt you want to actually be using mortars unless you want to go through artillery crew on a daily basis.

Early mortars were inefficient at the best of times and extremely dangerous at the worse. The fact that the GM said most gunpowder is old, wet and in otherwise not top quality would suggest that trying to do use mortars would be exceedingly hazardous to one's health and the health of everyone in the nearby area. You're honestly better off just have a catapult or trebuchet with firebombs since at least operating a catapult won't result in the machine itself exploding half the time.